A dirt jumping fork is a beefier fork all the way around. The crown might have a bit more material in it; most will have a beefier dropout area for supporting a 20mm thru-axle; the stanchions, although outwardly the same diameter as lighter models, may be of heavier-walled tubing; and it might have a chromoly or heavier-walled-aluminum steerer.

It's usually the bottom bushing slamming into the bottom-out bumper. To answer ken's question: it's called coil-bind; but I don't think there is enough preload adjustment on a fork to shorten one up to the point of coil-bind. I think it may be possible on some rear shocks to do that but I've never encountered that happening on a fork. And, yaeh, the best way to accomodate for weights is to change the spring - - there's not enough range in the preload to overcome too heavy a rider on too light a spring.