This is purely anecdotal and not tested (I will test when I have slightly more time myself), but don't size 4 units have significantly more trouble trampling size 3 units over size 2 ones? WingedDog mentioned the manual factors total unit size in damage calculations, but is it possible it's size difference that determines the damage?
Again, I've just noticed in my games and read in some AAR reports that size 3's don't die so quickly against chariots and such. Of course that could be accounted for by other stats of the units in question.
So who's gonna save me some testing time?