I thought the same thing, some freakishly large traps.
It's just in the mma world, just because he never tested positive, he is labeled as a natural athlete who does not use PED's on the other hand Alistair Overeem is a steroid monster. Overeem maybe a steroid monster, but that does not make Lesnar natural, it may only make him great at cycling things out of his body before testing.
Also if you look at his WWE pictures and mma pictures you can easly see that he lost almost half of his size, vascularity, hardness, even his skin color whitened

. Dude's body cant support the amount of muscle he had before without hardcore roid use i guess. And probably he didint had to take anything out of his system for WWE.
Those are my observations.