Beamer paper size is smaller than usual paper size.
The “paper size” of a beamer presentation is fixed to 128mm times 96mm.
With this trick, the normal font can be shown nicely.
But be cautious setting the image size with absolute value.
or use
\includegraphics[width=0.6\textwidth]{graphics.pdf}
http://web.mit.edu/8.13/presentations/beameruserguide.pdf
Citation
Beamer paper size is smaller than usual paper…
