same prog, different sizes

I just noticed that the same program compiled with the same flags (in
particular, march=i686) have *very* different sizes, both before and
after stripping. On a prescott and a ION atom, about 4K. On a core i3,
about 8K.

I expected differences (prescott and atom always yielded different
sizes, slightly smaller on prescott), but this seems absurd. Or is it?
As you guess, I've not been touching very modern hw. Is this something
to be expected? Am I doing something wrong?

In the core i3 I have gentoo; I replaced stable gcc (4.7*) by 4.8.3,
and it got better, but not much. (I also recompiled dietlibc with gcc
4.8, just in case)  On the atom and the prescott, Slackware 14.1 (gcc
4.8.2).
RAM (in case it matters):
atom 4G
prescott 2G
i3 8G


Any one with this experience?

Thanks