I'd have said the same thing. That makes us both brilliant, right?
Seriously, Williams and Rivers are both kids and could put on 15-20 lbs. of muscle in a pro weight room. Marshawn Lynch played at 215. Not that these guys would ever be another Beastmode, but maybe Baby Beasts.
Lynch had incredible upper leg strength - could step out of tackles, run from a wide stance and make a cut by firing one thigh, and continue to churn his legs when seemingly stopped - and surprise announcxers when he simply took off for more yards.
Thigh strength and explosion off the thighs can be developed by trainers. Larry Fitzgerald, about four years into his career, had thigh strength and explosion rarely seen in a wideout - and he developed it with the Cardinal's training staff.