Font size different in \newtheorem heading and body text versus the rest of the document

Question

I am using the book document class with no font size specified, so it should be the default 10pt.

After \begin{document} I have the line \fontsize{9}{10.8} to change all the font to 9pt.

\documentclass{book}

\usepackage{bookman}
\usepackage{amsthm}

\newtheoremstyle{break}
  %{\topsep}{\topsep}%
  {8pt}
  {8pt}
  {}{}%
  {\bfseries}{}%
  {\newline}{}%
\theoremstyle{break}
\newtheorem{test}{Test}
\counterwithin*{test}{subsection}

\begin{document}

\fontsize{9}{10.8}

The quick brown fox jumps over the lazy dog. This font looks to be 9pt.

\begin{test}
This is the test.  This seems to come out smaller.
\end{test}


\end{document}

The heading "Test" and the body text are smaller than the 9pt specified in \fontsize.

What smaller font size is this? And can I get the entire document to output in this smaller font size with \fontsize? Or is \fontsize causing these two sizes of fonts?

Answer 1

The declaration \fontsize{9}{10.8} doesn't take effect until \selectfont is issued. This happens in the theorem, but not in the text above it.

\documentclass{book}

\usepackage{bookman}
\usepackage{amsthm}

\newtheoremstyle{break}
  %{\topsep}{\topsep}%
  {8pt}
  {8pt}
  {}{}%
  {\bfseries}{}%
  {\newline}{}%
\theoremstyle{break}
\newtheorem{test}{Test}
\counterwithin*{test}{subsection}

\begin{document}

\fontsize{9}{10.8}\selectfont

The quick brown fox jumps over the lazy dog. This font looks to be 9pt.

\begin{test}
This is the test.  This seems to come out smaller.
\end{test}


\end{document}

On the other hand, using a specialized class for this seems better.

\documentclass[9pt]{extbook}

\usepackage{bookman}
\usepackage{amsthm}

\newtheoremstyle{break}
  %{\topsep}{\topsep}%
  {8pt}
  {8pt}
  {}{}%
  {\bfseries}{}%
  {\newline}{}%
\theoremstyle{break}
\newtheorem{test}{Test}
\counterwithin*{test}{subsection}

\begin{document}

The quick brown fox jumps over the lazy dog. This font looks to be 9pt.

\begin{test}
This is the test.  This seems to come out smaller.
\end{test}


\end{document}