So during Alpha I heard a few rumblings that the F18 minimal install was very large. Having a few minutes free this afternoon I thought I'd compare F17 and F18 minimal installs. I did a minimal install from the F17 DVD, then a minimal install with the latest F18 pre-beta smoketest image (from online repos), then looked at the package lists and space taken by the install. Result is that F18 minimal install is in fact smaller than F17's: it installs fewer packages and uses less space.
F18 minimal install installs 194 packages and uses 723MB in / and 44MB in /boot.
F17 minimal install installs 209 packages and uses 821MB in / and 55MB in /boot.
So it looks like at least as of now, F18 is doing pretty well on this front.