As scary as this sounds, I do agree with you (that might be a first!) that Bryant will be a better defender than either the 6'6" guy or HMP. HMP was not very well coached in fundamentals. That happened before he came to IU, and IMHO, Crean and his staff did little to improve his skills. In theory, a 3-year player should be better than a freshman. HMP had good physical skills. Bryant will come in the door with better fundamentals and more basketball IQ.
Better defensive teams force their will on the other team. They force the ball to go to certain players, or they force their opponent into taking certain shots. Of course "forcing" is not always 100% acheived, but you get the idea. It is more than 1-on-1 guarding of your player. They play a "team defense". I think IU will have shorn up their primary defensive weakness in the center. However, I still have doubts that IU will play an over-all better team defense. Maybe they can improve just enough that their offensive power can overcome the defensive weakness. I think that is the best you can hope for.