You need to provide mathjax declaration for the \mathlarge command. It can be done using the following configuration file:
\RequirePackage{mathjax-latex-4ht}
\Preamble{xhtml}
\begin{document}
\Configure{@HEAD}{\HCode{\unexpanded{<script type="text/x-mathjax-config"> MathJax.Hub.Config({ TeX: {Macros : { mathlarger: ["{\\large \#1}",1] }}});</script>}}}
\EndPreamble
It uses mathjax-latex-ht package from the helpers4ht bundle for MathJax loading. The declaration is added using:
\Configure{@HEAD}{\HCode{\unexpanded{<script type="text/x-mathjax-config"> MathJax.Hub.Config({ TeX: {Macros : { mathlarger: ["{\\large \#1}",1] }}});</script>}}}
This is configuration code for MathJax, you can use commands supported by MathJax to change the font size.
This is the result:

I've used a following mwe:
\documentclass{article}
\usepackage{amsmath}
\usepackage{relsize}
\begin{document}
\begin{equation}
P_{a}(x)\cdot P_{b}(x)=\mathlarger{\int}_{-1}^{+1}\,P_{a}(x)\,P_{b}(x)\,dx
\label{eq18}
\end{equation}
\end{document}
\mathlargefor mathjax – michal.h21 Jun 01 '18 at 10:34\bigintssss,\bigintsss,\bigintss,\bigints, and\bigint? – Mico Jun 01 '18 at 11:22