Reduce font size within all proofs

Question

I'm working on some notes with plenty of proofs. I'd like to reduce the font size of the proofs, while holding constant the size of the rest of the text.

Here is some code that achieves this by manually changing font size in each proof. But there are many proofs in the document; I'm looking for a way that I can apply this change to all proofs, without having to manually change size in each proof individually.

\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem{theorem}{Theorem}

\begin{document}
\begin{theorem}
This is my theorem. 
\end{theorem}
\footnotesize
\begin{proof}
Here is a small proof. 
\end{proof}
\normalsize 
\begin{theorem}
This is a second theorem.
\end{theorem}
\footnotesize
\begin{proof}
And another proof. 
\end{proof}
\end{document}

Answer 1

Welcome! Use \AtBeginEnvironment in your preamble:

\documentclass{article}

\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem{theorem}{Theorem}
\AtBeginEnvironment{proof}{\footnotesize}

\begin{document}

\begin{theorem}
This is my theorem.
\end{theorem}
\begin{proof}
Here is a small proof.
\end{proof}
\begin{theorem}
This is a second theorem.
\end{theorem}
\begin{proof}
And another proof.
\end{proof}

\end{document}