Same here 6'6" and working hard to get rid of the skinny legs. I've been told "No white boy like you should have an ass like that!" by one of the nurses I used to work with.
Part of it may be in how we walk and climb stairs. Tall guys learn to walk next to shorter people so we moderate our stride. In doing so some of us use more ass (swing the leg straighter from the hip) than thigh/knee to walk.
8 inch stairs only cause us to lift our leg a little due to long levers moving farther at their end points. A shorter leg will have to be lifted higher and more thigh recruitment due to a smaller knee and hip angle like doing a deeper one legged squat.
With these two hypotheses some tall guys develop more ass because of how they walk.
Any exercise physiologists out there have any ideas?