1) Yeah, they're undersized. That's their thing. They have been undersized all season and they were undersized when they won the title last year. They play a style of small ball that's almost impossible to stop, high screen and roll with Lebron/Wade/Chalmers and a Bosh pick, Bosh will pop and Ray Allen and Shane Battier will be in the corners. If you help on the ball-handler coming off the pick then you leave one of the three point shooters open, if you switch on the pick then the ball carrier is guarded by a big man and can drive or they can kick to the small man on Chris Bosh, who's one of the best mid-range shooters in the game. It's a pick your poison scenario.
2) Um what? Firstly, Indiana have one centre, Roy Hibbert, and at his absolute best he's probably the 5th best centre in the league. What? :S
3) Roy Hibbert is a good defender and has lessened his tendency to get into foul trouble this season significantly, which is a very good thing. David West is strong, but a pretty average defender and Paul George just got decimated in the post by Lebron. Indiana are a good defensive unit but the Heat are an amazing offensive force, as good if not better on offense as Indiana are on defense.