Height alone means nothing. What matters is standing reach. Powe has very long arms.
Perkins also has very long arms, which is one reason why he's a good shot blocker even though he's not a great leaper.
BBD does not have long arms. While is a more skilled all-around player than Powe, BBD has still not learned how to upfake, absorb contact and finish at the rim. He has to learn to shoot when the defender is on the way down, not on the way up.