Theory #1 of why Brad has no Ass: The taller you are the smaller your ass is.

Taller people require less work when taking stairs or performing similar actions, hence taller people don't work their asses as much as shorter people, making shorter people have more powerful asses than taller people. Another component is that the taller you are, there is more volume for your ass to exist over. A tall person may have as much muscle/flab as a short person but since it is spread out it is not very noticable or bubbly. If one was to take the ass of a tall person and put it in the space of a short person it would be quite noticable.

This theory can further be backed by observatiobal evidence. I at 6'6" have no ass but I have friends who are between 5' and 5'4" who have "explosive asses" or consider themselves to be "ass models".