I'll the him answer on his own, but my $0.02 in case he doesn't come back around for a while.
You're looking at this the wrong way..
You are correct that a single molecule of Cr2O3 will not change it's size, weight, etc, etc.
However, a single molecule of CrO2 is not 0.5 microns. A 0.5 micron grain of Cr2O3 contains a HUGE number of molecules. What we see as powder, is in fact a pile of Cr2O3 crystals. Those crystals could have been milled down to any size.