I have built a transition in AE to drop into a FCP 7 timeline. The source of the images used for the transition are still frames generated from FCP and then exported as PNG and imported to AE. In AE the first image is scaled up to match the second and has to match scale at the first frame or edit following the transition.
Although the export settings from AE match that of the FCP sequence, when I bring the AE-generated footage in it appears slightly larger than the FCP frame. I don't understand why.
I know the recent versions of AE have changed the aspect ratio of DV and HDV 16:9 footage from 1.2 to 1.212. Could that be the issue? Check the file properties. You may need to render out using square pixels and interpret the footage in FCP to 1.2 AR.