I agree. A lot of linebackers and elite defensive ends have the same vertical jumps as defensive backs. It's there massive lower body strength that gives them so much power to jump high. But, in the 40, they can't keep up with the little guys because it's hard to have fast top end speed when you weigh 250 lbs. Extra body weight takes away from your reactivity, which will effect how "springy" you are once you've reached top speed. The more springy you are the more ground you are covering with each step you take.
Good reply.
They also probably have a very strong strength background and they're used to muscle up everything which works well in terms of acceleration but when you get to the relaxation/reactive sprinting they will suck hard there trying to force their sprint up again and again instead of working to maintain a high speed.
Not sure if 40 yards is a long enough distance for that to happen though.