Plus the macbooks screen has enough resolution for 720i/p video to be displayed fully.
Semantics aside, it's not HD.
But in order for blu-ray movie playback to be practical it must be 1080p. 720 ≠ 1080.
I have a 26" 720p tv that downscales 1080i input for display. I have the same 1080i input on my 52" 1080p in the living room. There is no question, the 52" is sharper and clearer. In order for HD to be practical, both the source and the display must match resolution. Size has nothing to do with it.