First of all I don't know what their starting heights and weights are coming into UConn and how much they've changed since but everyone carries their weight differently and reacts to putting on muscle differently. Just by looking at them Bouknight has what looks to be a classic ectomorph body type, long limbs with smaller joints/smaller waist/rib cage. Adams has more of a mesomorph body type, thicker structure/sturdier...My guess is neither of them did any serious lifting or took their eating seriously before coming to UConn...Adams has the type of frame that will most likely respond better to weight training in terms of putting on size and muscle. It doesn't mean James can't get as strong as him but he would most likely have to strength train a lot harder and eat a lot more to keep up with Adams. They are just different type of builds and different types of athletes.
You can see it with your eyes by looking at their bodies, you can also see it in how they move. Adams is much more rigid, James glides around the court and then can explode in an instant. I would pretty much guarantee if you looked at their ankles Bouknights are small and Adams are rather thick. Most of your truly great basketball athletes are ectomorphs. Mesomorphs can be good basketball players too but they tend to be more inclined for rugby, football, powerlifting/bodybuilding.