Agree...but it is a deficiency. Football is like a chess game, you move to reinforce one area, you make the other weak. So, if he does have to hold a block longer, and cannot, you have to gameplan around that. Again, nothing against the player, but there is a reason why a 5-8 guy is never going to play LT. While those reasons are watered down, the physics hold true for a RB as well. If I've got a 220 lb back and a 190 lb back with everything else being equal, I'll take the 220 lb kid seven days a week and twice on Sunday.
Sanders will most likely carry the majority of the load,unless he gets hurt. Allen is a backup, and will spell Sanders at times at most.