One of this key weapons is his ability to be mobile and buy time to attempt to make plays. Getting bulked up isn't really going to help that ability and it's never going to result in him being a better athlete or strong enough to shed defensive linemen.
He just is what he is.
IMO, his physical abilities(or lack thereof) are actually less concerning than his mental makeup, his dedication to correcting his mechanical flaws and his reading abilities. Those he was supposed to be elite at and he is dramatically subpar.