What I've observed is that, with egcs (old toolchain) on i386, you
normally see:

	-Os < -O < -O2

but you sometimes see:

	-O < -Os < -O2

-Os turns off certain optimizations which tend to bloat code size and
increase instruction count; it turned out that, for i386, it reduced
instruction count but increased object code size for the one case I
looked at in detail.

The differences were often small, but with our install media commonly
very close to the limit, a few bytes can make a difference..