I'm running Mac OS X 10.1.3, and I cannot get anything to run full screen in QuickTime and have it play smoothly.
I thought it was because I was making it too large (my screen res is 1600x1200). What I did to verify this was I played back a 320x240 MPEG movie at 640x480 at my current screen res of 1600x1200. It played back smoothly as expected.
Then I proceed the test by lowering my screen res to 640x480. When I set it to play at full screen it was totally choppy! Then I reset it to double and played it back, and again it was totally choppy!
To extend my test I switched my screen res to 800x600 and played back the movie at double, and it was smooth all the way. Then I played the movie back at full screen at 800x600, and yet again it was choppy beyond belief.
What I have conclude is that when ever a movie, MPEG, or MOV, or any other format is played back at the same res as the screen (i.e. doubled, or full screen) it becomes choppy. I don't know if this is true for all of you or not, but it sure is for me.
-dantjie