The very definition of average means middle or intermediate meaning you must have a floor and ceiing in order to have a middle
Any normal distribution is a Bell Curve. For every Brandon Jacobs you have a Darren SProles. Same can be applied to the RB entering the NFL draft, For every Turbin who may be at one end of the Bell Curve you have a LaMicheal James at the other. It's a fallacy to say every RB that enters the NFL will gain weight. It just is not true based on some average statistic.