Centering captions in document that contains A3 and A4 paper sizes

Question

In order to fit multiple figures on one line, my document has A4 (portrait) and A3 (landscape) pages. To set up the A3 pages I created a new environment (a trick I adapted from another solution). Please note that I can't create a separate pdf for the A3 pages and add it in because figure references, pages, sections etc. will be constantly changing.

However, when I plot the figures on the A3 page, the main caption is placed as if it was A4. (See below) How do I center the caption relative to the A3 page?

I include the code without the text in the A4 sections:

\documentclass[12pt]{article}

% Pictures & Labels
\usepackage[capitalise]{cleveref}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{textcomp}
\usepackage{float}
\usepackage[section]{placeins}
% Paper Style
\usepackage[a4paper,
            width=150 mm,
            top=25 mm,
            bottom=25 mm]
            {geometry}
\usepackage[title,toc,page]{appendix}
%% Allow A3 sheets - - New environment
\newenvironment{a3page}
{\newpage
\advance\textwidth\pdfpagewidth
\headwidth = \textwidth 
\pdfpagewidth=2\pdfpagewidth}
{\newpage}

\begin{document}

%% Start A3 page
\begin{a3page}
\section{Title}
\begin{figure}[H]
\centering
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
                \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic.png}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.05\textwidth}
                \centering
                \includegraphics[height=4cm]{images/pic2.png}
        \end{subfigure}%
        \centering
        \caption{This caption isn't centered}\label{fig:}
\end{figure}
%
%
\end{a3page}
\end{document}

Answer 1

There is more to a page layout, then just the well known parameters like \textwidth. For example, internally TeX uses \hsize for building paragraphs. The best is, to leave this to the geometry package.

Unfortunately, \newgeometry doesn't allow to switch between portrait and landscape or change the paper size mid document. But a macro can be written, which allows this. It's basically a changed copy of \newgeometry, called \newgeometryfull here. Use it with care! Additionally, \pdfpageheight and \pdfpagewidth must be set, so the PDF viewer also knows the paper size. This is done in the a3page environments.

I removed the line \headwidth=\textwidth, because \headwidth doesn't exist in your example. But may be you need it for headers not present here.

The result:

The code:

\documentclass[12pt]{article}

% Pictures & Labels
\usepackage[capitalise]{cleveref}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{textcomp}
\usepackage{float}
\usepackage[section]{placeins}
% Paper Style
\usepackage[a4paper,
            width=150 mm,
            top=25 mm,
            bottom=25 mm]
            {geometry}
\usepackage[title,toc,page]{appendix}

\usepackage{lipsum}

\makeatletter
% like \newgeometry, but also allows change of landscape/portrait and paper size
% to be used with caution!
\newcommand{\newgeometryfull}[1]{%
  \clearpage
  \Gm@restore@org
  \Gm@initnewgm
%  \Gm@newgmtrue
  \setkeys{Gm}{#1}%
%  \Gm@newgmfalse
  \Gm@process
  \ifnum\mag=\@m\else\Gm@magtooffset\fi
  \Gm@changelayout
  \Gm@showparams{newgeometry}}%
\makeatother

%% Allow A3 sheets - - New environment
\newenvironment{a3page}{%
    \newgeometryfull{a3paper,landscape,width=360 mm,top=25 mm,bottom=25 mm}
    % set the correct dimension for the PDF viewer
    \pdfpageheight=\paperheight
    \pdfpagewidth=\paperwidth
}{%
    \restoregeometry
    % set the correct dimension for the PDF viewer
    \pdfpageheight=\paperheight
    \pdfpagewidth=\paperwidth
}

\begin{document}
\section{A test}
\lipsum[1]

%% Start A3 page
\begin{a3page}
\section{Title}
\lipsum[1]

\begin{figure}[H]
\centering
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:a}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:b}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:c}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:d}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:e}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:f}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.14\textwidth}
                \centering
                \includegraphics[height=4cm]{example-image-9x16.png}
                \caption{}
                \label{subfig:g}
        \end{subfigure}%
        %
        \caption{This caption isn't centered}\label{fig:}
\end{figure}

\lipsum[1]
\end{a3page}

\section{Another test}
\lipsum[1]

\end{document}

Answer 2

This is my not so elegant solution. I used

\eject \pdfpagewidth=16.5in \pdfpageheight=11.7in

To make the page have different size. This solution is based on this answer. Now the caption gets centered.

\documentclass[12pt]{article}

% Pictures & Labels
\usepackage[capitalise]{cleveref}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{textcomp}
\usepackage{float}
\usepackage[section]{placeins}
% Paper Style
\usepackage[a4paper,
            width=150 mm,
            top=25 mm,
            bottom=25 mm]
            {geometry}
\usepackage[title,toc,page]{appendix}
 \begin{document}
A4 paper

%% Start A3 page
\eject \pdfpagewidth=16.5in \pdfpageheight=11.7in
\section{Title}
\begin{figure}[H]
\centering
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
                \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.16\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
                \caption{}
                \label{subfig:}
        \end{subfigure}%
        %
        %
        \begin{subfigure}[t]{0.05\textwidth}
                \centering
                \includegraphics[height=1cm]{example-image-b}
        \end{subfigure}%
        \centering
        \caption{This caption isn't centered}\label{fig:}
\end{figure}

\end{document}