I saw Cox play twice at Yazoo and watched him one day at a camp. At 6'4", 238 (which looked accurate) I thought he had incredible speed coming off the end compared to other recruits.
lots of guys that are naturally 230 - 250 jump up 20 or 30 pounds just because of having a meal plan to use and lifting weights like they do. Most 18 year olds don't know how to eat and don't have anyone breathing down their neck in the weight room.
Best example I saw in person was Q. Wesley. He went from I want to say 230 or so to up around 290 300. The guy said he never got 3 square meals a day and never lifted a weight before he got to Starkville.