How to adjust the size of the subscript

Question

In the Tikz environment the subscripts are appearing in the same size as the scripts themselves, How can I fix this, please? The code I am using is the following

\documentclass[tikz,border=2mm]{standalone}
\usepackage{amsmath}
\begin{document}
\begin{tikzpicture}[line cap=round]
\node at (-4.2,0){\tiny $a_0$};
\end{tikzpicture}
\end{document}

Answer 1

There's nothing wrong with this. It may be that the lowercase a is smaller, causing your illusion.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\begin{document}
$a_0$ $A_0$ $A_a$ $a_a$
\begin{tikzpicture}
\node at (-4.2,0) {$a_0$ $A_0$ $A_a$ $a_a$};
\end{tikzpicture}
\end{document}

Add

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\begin{document}
$\displaystyle a_0$ $\textstyle A_0$ $\scriptstyle A_a$ $\scriptscriptstyle a_a$
\begin{tikzpicture}
\node at (-4.2,0) {$\displaystyle a_0$ $\textstyle A_0$ $\scriptstyle A_a$ $\scriptscriptstyle a_a$};
\end{tikzpicture}
\begin{tikzpicture}
\node at (-4.2,0) {$a_{\scriptscriptstyle 0}$ $\textstyle A_0$ $\scriptstyle A_a$ $a_{\scriptscriptstyle a}$};
\end{tikzpicture}
\end{document}

Answer 2

It has nothing to do with the tikzpicture. You should use e.g. \scriptstyle instead of \tiny for math - see e.g. Reducing font size in equation

\documentclass{article}
\usepackage{tikz}
\begin{document}
~\\
\begin{tikzpicture}[baseline, inner sep=0pt, outer sep=0pt]
\node[anchor=base] at (-4.2,0){\tiny $a_0$};
\end{tikzpicture} - TiKZPicture\\
{\tiny $a_0$} - tiny size\\
$a_0$ - normal size
\end{document}