TeX capacity exceeded in figure captions

The problem is the {\clist_item:Nn \l_tmpa_clist {1}}\textwidth in my point of view, it does not expand to the correct dimension (multiple).

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{xparse}
\usepackage{subcaption}
\usepackage{graphicx}

\ExplSyntaxOn
\clist_new:N \l__gerry_mycl


\cs_new:Nn \__gerry_customfiggg:nnnn {
  \includegraphics[width=#1\textwidth]{#2}
  % caption produces tex capacity exceeded.
  \caption[#3]{#4}
  \label{fig:#2}
}

\NewDocumentCommand{\multifig}{m}{
  \begin{figure}
    \clist_set:Nn \l__gerry_mycl {#1}
    \clist_map_inline:Nn \l__gerry_mycl {
      \clist_set:Nn \l_tmpa_clist {##1}
      \begin{subfigure}{\clist_item:Nn \l_tmpa_clist {1}\textwidth}
        \__gerry_customfiggg:nnnn {0.95}{\clist_item:Nn \l_tmpa_clist {2}}{\clist_item:Nn \l_tmpa_clist {3}}{\clist_item:Nn \l_tmpa_clist {4}}
      \end{subfigure}
    }
  \end{figure}
}
\ExplSyntaxOff



\begin{document}

% size, file name, caption, caption
\multifig{{.2, ente, small caption 1, big caption 1}, {.1, ente1, small caption 2, big caption 2}}

\end{document}

Your internal function should be named differently, as it has four arguments.

Also, \exp_args:Nx will not do what you expect, because it just expands the contents of the first braced group after the next token. Actually, this is not really needed, but I left it just by way of example.

You should define a variant that expands all arguments. By the way, you need no clist variable.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{expl3,xparse}
\usepackage{subcaption}
\usepackage{graphicx}

\ExplSyntaxOn
\cs_new:Npn \__gerry_customfiggg:nnnn #1#2#3#4
 {
    \includegraphics[width=#1\textwidth]{#2}
    \caption[#3]{#4}
    \label{fig:#2}
 }
\cs_generate_variant:Nn \__gerry_customfiggg:nnnn { xxxx }

\NewDocumentCommand{\multifig}{m}
 {
    \begin{figure}
    \clist_map_inline:nn { #1 }
     {
        \begin{subfigure}{\clist_item:nn {##1}{1}\textwidth}
        \__gerry_customfiggg:xxxx % \__gerry_customfiggg:nnnn would do as well
          {0.95}
          {\clist_item:nn {##1}{2}}
          {\clist_item:nn {##1}{3}}
          {\clist_item:nn {##1}{4}}
        \end{subfigure}
}
\end{figure}
}
\ExplSyntaxOff



\begin{document}

% size, file name, caption, caption
\multifig{
  {.2, example-image-a, small caption 1, big caption 1},
  {.1, example-image-b, small caption 2, big caption 2}
}

\end{document}

On the other hand, I'd favor a completely different and much more flexible approach:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{expl3,xparse}
\usepackage{subcaption}
\usepackage{graphicx}

\ExplSyntaxOn
\keys_define:nn { gerry/multifig }
 {
  align .tl_set:N        = \l_gerry_multifig_align_tl,
  scale .tl_set:N        = \l_gerry_multifig_scale_tl,
  innerscale .tl_set:N   = \l_gerry_multifig_innerscale_tl,
  caption .tl_set:N      = \l_gerry_multifig_caption_tl,
  shortcaption .tl_set:N = \l_gerry_multifig_shortcaption_tl,
  label .tl_set:N        = \l_gerry_multifig_label_tl,
  align .initial:n       = c,
  scale .initial:n       = 1,
  innerscale .initial:n  = 0.95,
 }
\NewDocumentEnvironment{multifig}{O{htp}}
 {
  \begin{figure}[#1]
 }
 {
  \end{figure}
 }

\NewDocumentCommand{\shortfig}{mm}
 {
  \group_begin:
  \keys_set:nn { gerry/multifig } { #2 }
  \begin{subfigure}[\l_gerry_multifig_align_tl]{\l_gerry_multifig_scale_tl\textwidth}
  \centering
  \includegraphics[width=\l_gerry_multifig_innerscale_tl\textwidth]{#1}
  \tl_if_empty:NTF \l_gerry_multifig_shortcaption_tl
   {
    \caption{\l_gerry_multifig_caption_tl}
   }
   {
    \caption[\l_gerry_multifig_shortcaption_tl]{\l_gerry_multifig_caption_tl}
   }
  \tl_if_empty:NF \l_gerry_multifig_label_tl { \label{\l_gerry_multifig_label_tl} }
  \end{subfigure}
  \group_end:
 }
\ExplSyntaxOff



\begin{document}

\ref{examplelabel}

\begin{multifig}
\shortfig{example-image-a}{
  scale=.2,
  shortcaption=small caption 1,
  caption=big caption 1,
  label=examplelabel,
}\qquad
\shortfig{example-image-b}{
  align=t,
  scale=.2,
  innerscale=.5,
  caption=caption 2,
 }
\end{multifig}

\begin{multifig}
\shortfig{example-image-a}{
  align=t,
  scale=.2,
  shortcaption=small caption 1,
  caption=big caption 1,
}\qquad
\shortfig{example-image-b}{
  align=t,
  scale=.2,
  innerscale=.5,
  caption=caption 2,
 }
\end{multifig}

\end{document}