Well perhaps he used the marijuana to gain mass
LOL. But seriously, it's unlikely he tested for weed. It's far, far more likely it was a banned substance that has to do with gaining mass or some kind of steroid. Doesn't make sense to remain hush, hush about weed. I think the chances that was the substance is almost zero.
You generally don't lose much respect for testing for weed, so really no reason not to cop to it. But I did lose respect for him after his test since I know it wasn't something as petty as weed.