As I understand it (it’s been a while since I’ve worked on the Mac), when using quicktime to view videofiles, it converts them for you in order four you to have good playback. So in essence, your not looking at your final render, but at a compressed version of it (I’d presume h264-mp4, or the likes).
I’d recommend using a different mediaplayer for reviewing your final renders (VLC, QT7, Catalyst…)