Hi,
welcome to the board!
I can see the problem, it's designed this way and I don't like the size relation too.
Code: Select all
\documentclass{slides}
\usepackage[T1]{fontenc}% similar result also without
\begin{document}
m$m$m
\end{document}

- slides-fonts-default.png (1.4 KiB) Viewed 7898 times
You could use the command
\DeclareMathSizes
to adjust it. The class calls
Code: Select all
\DeclareMathSizes{19.907}{19.907}{16.59}{13.82}
and for further values. It can be changed, for example:
Code: Select all
\documentclass{slides}
\usepackage[T1]{fontenc}
\DeclareMathSizes{19.907}{21.9}{16.59}{13.82}
\begin{document}
m$m$m
\end{document}

- slides-fonts-adjusted.png (1.5 KiB) Viewed 7898 times
However, it will also affect capital letters, so if you would use capital letters in math mode, you would see they are too big then. It's just that the sizes of small letters in the default choosen fonts don't match well, but the capital letters do. You could consider to use different fonts.
Finally, I support Thorstens recommendation to choose a more modern class. Such manual fixes are not recommendable. The mentioned classes
beamer
and
powerdot
are well designed and offer much more features.
Stefan