I think mingw only has a static-linked runtime library, so you will be linking items of that into the dlls increasing size.
As for the options mentioned:
#include <cstdio>
makes sense if nothing in <iostream> is used
<iostream> usually builds on <cstdio> so they would both be included, increasing code-size.
Stripping symbols - good idea but only for release mode (and true release mode at that, not a 'release' where you are still debugging but are linking against the release libraries). The symbols are what debuggers etc. use to give better info about where a crash occured.