I recommend to avoid using figure environments in beamer, because usually the figures are not intended to float in presentations, and they don't do it as Herbert commented. It can avoid confusion with floats in standard LaTeX. If you just use
figure because you need captions, you could do it with the
\captionof command of the
caption package instead.
Here's an example, where a
minipage of a small size
.3\textwidth is used together with a caption, and enlarged by
\textwidth, the whole box including the caption is scaled.
It would cause an error if
\caption would be used here instead of
minipage. Note, by using the
caption package you can benefit from further features of this great package.