Clothing size does not necessarily account for being overweight for two major reasons: clothing cut does not suit everyone's body proportions, and clothing sizes are not consistent.
Case in point: the girl who has really big boobs and is forced to wear size 16 tops to accommodate them compared to her size 12 bottoms... or go into topshop and buy a pair of trousers which are 2 sizes bigger than the normal sizing because topshop's sizing is out of whack. There you are banning two completely different people because of their clothing size not because of how overweight they are.