Some more information to maybe make more sense of my original question!
The timeline in Avid was 720×486. I had my editor output the movie at 648×486 because my ultimate goal was to get the movie into an 864×486 AE comp, so as to create a movie to play back on a widescreen DVD. On a 4:3 monitor, it would play back normally; on a 16:9 monitor, you’d have letterboxing (pillarboxing?) on the side.
When I look at the info file in Quicktime, the info says that the movie has a normal size of 720 x 486 and a current size of 648×486. Is that my problem? Is the file really a 720×486 movie and it’s trying to squash it to fit the 648×486?
If so, how do I fix/change that? Thanks again.