In old time football, the O-linemen were 40 lbs heavier than the D-linemen. They could physically impose themselves on the D-line and wear them down. Throw in a big back and the wear increases. TO did this and almost always owned the 4th quarter.
That weight imbalance is not true today. D-linemen now are generally just as big and more athletic than the O. Today they try to wear the D by using a fast pace. Get them gassed and don't let them catch their breath.
In short, it does not matter how big your back is if the line can't open holes for him to run through. What Robinson has going is he is small and quick. I've seen him jump through some holes that a cat would have trouble squeezing through. A big back would have been stuffed.
IMHO Mo Washington is exactly what this team needs in a RB. He is strong enough to break tackles, but more importantly he is shifty and fast. He is mobile enough to pick a new hole if the intended hole is not there. He has good hands and can get to the corner.