I notice he always wears tight-fitting black shirts that emphasize his compact frame. Jay used to have that small-man wardrobe too, always shopping in sleeveless tank tops at the local Costco. Seems the guys most secure about their size tend to stick to sweaters and long-sleeve shirts/pants.
There's no doubt Heath is extremely muscular, but he just doesn't have the skeletal development of a Mr. O. If you were to post a most muscular picture of the last three Mr. Os (other than Dexter), you'd notice their shoulders jutting out above their upper arms. Phil's shoulders are behind his outer arms, in a kind of pyramid configuration. Sucks.