Trouble changing font size in figure legends

Question

I'm trying to set the font size smaller in the legends for all of my figures and tables. The way I was trying to do this was using \AtBeginEnvironment but that doesn't seem to do anything at all. This is a minimal example of what is failing:

\documentclass[a4paper, 12pt]{report}

\usepackage{etoolbox}
\usepackage{relsize}

\AtBeginEnvironment{figure}{\smaller}

\begin{document}
\begin{figure}
\caption{whatever}
Text
\end{figure}

Outside text
\end{document}

Now that should, I think, make 'Text' appear smaller than 'Outside text' exactly as it would if you put the \smaller inside the figure environment. But it doesn't. I tried using the floatrow package instead but it messes up the ordering of the figure caption and the legend text and \captionsetup will only alter the caption itself not the legend text.

Any help? Why does \AtBeginEnvironment not work?

Answer 1

One of the last things the figure enviroment does is issuing the internal command \@floatboxreset whose definition is

\def\@floatboxreset{%
  \reset@font
  \normalsize
  \@setminipage
}

and this is why the \smaller declaration has no effect, because the tokens in \AtBeginEnvironment are found earlier.

You should instead change the meaning of \@floatboxreset

\makeatletter
\AtBeginEnvironment{figure}{%
  \def\@floatboxreset{\reset@font\small\@setminipage}%
}
\makeatother

The change will not leak outside the environment.

In case you're using setspace, something more should be done, as the package patches \@xfloat to issue again \normalsize, which is an error, in my opinion, as \selectfont would suffice.

\documentclass[a4paper, 12pt]{report}

\usepackage{etoolbox}
\usepackage{setspace}

\makeatletter
\AtBeginEnvironment{figure}{%
  \def\@floatboxreset{\reset@font\small\@setminipage}%
}
\patchcmd{\@xfloat}{\normalsize}{\selectfont}{}{}
\makeatother

\begin{document}
\begin{figure}
\caption{whatever}
Text
\end{figure}

Outside text
\end{document}

Answer 2

One way might be to redeclare the @makecaption command.
First we define the font and size for the caption-Title (figure or table) and the font for the caption-text. Then \@makecaption is rewritten:

\documentclass[]{article}
\usepackage{mwe}
\usepackage{graphicx}

\newcommand*{\textCapTitFont}[1]{\textbf{#1}}  
\newcommand*{\Captionfont}{\small\sffamily}
\makeatletter
    \long\def\@makecaption#1#2{%
      \vskip\abovedisplayskip\Captionfont%
      \setbox\@tempboxa\hbox{\textCapTitFont{#1}~~#2}%
      \ifdim \wd\@tempboxa > \hsize
        \textCapTitFont{#1}~~#2\par                  
      \else
        \hbox to \hsize{\hfill\box\@tempboxa\hfill}%
      \fi
      \vskip\belowcaptionskip}
\makeatother

\begin{document}
Some normal text, not beeing in a caption, just to compare 
\begin{figure}\centering
\includegraphics[width=0.5\linewidth]{example-image-a}
\caption{This is an explanation \ldots}
\end{figure}
\end{document}

This looks like this: