The big bruising RB is a thing of the past. DL are so much bigger, stronger, and faster than they used to be that a 250 lb RB just isn't big enough to run through a 330 lb DT. That 250 lb RB has to be fast.
Remember from high school physic: Force = Mass x Acceleration. AJ Dillon isn't accelerating, so he's not generating a whole lot of force.
Just like AJ Hawks, Dillon's 40 is not representative of his game speed. Huge bummer. Let's hope Jones comes back soon.