960x720 ftg looks squooshed but ok in fcp

I'm working w/ some green screen ftg. Captured in Final Cut at HD (960x720) w/
DVCPro HD 720p 60 codec. I export the ftg., w/ no compression, million colors,
at current size, which strangely shows up as 637x358. I import that in AECS3. It
comes in at 960x720 but it looks squooshed. I key it and render it w/ animation
codec at 960x720. I import that back into final cut (or avid) and it looks fine.
This is how I have always done it.

The problem today is that I have to provide my client with a matte and a fill.
When I give them the rendered file from AE (QT file) it looks squooshed. The
client will use these files in flash which I know nothing about. They want to
know why the QTs I provided them look wrong. Can anybody explain?