%%
%% The LaTeX Companion, 3ed
%%
%% Example 7-4-16 on page I-557 in "Customizing the subcaptions".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{219.0pt}

\newcommand\samplefigs[1]{%
  \begin{subfigure}[#1]{14mm}  \centering
     \includegraphics[width=6mm]{cat}
     \caption{Small, but a large caption}
   \end{subfigure}\dots
   \begin{subfigure}[#1]{14mm}
     \includegraphics[width=14mm]{elephant}
     \caption{Bigger}
   \end{subfigure}}

%StartShownPreambleCommands
\usepackage{graphicx,subcaption}
% \samplefigs as defined before
%StopShownPreambleCommands

\begin{document}
\begin{figure}  \centering
 \captionsetup[sub]{skip=3pt} % for sub
 \captionsetup     {skip=5pt} % for main
  \subcaptionbox{top}   {\samplefigs{t}}
  \quad
  \subcaptionbox{bottom}{\samplefigs{b}}
  \caption{Different alignment options}
\end{figure}
\end{document}