what AE version are you using.
And what do you mean by smaller ?
If you are using AE CS4 and import your video, into a PAL composition, you may experience 2 thin black bars on the left and right side. That's because the Pixel Aspect Ratio have been corrected to reflect the clean aperture instead of the production aperture.
If you need to use the old PAR values, Chris & Trish Meyer have posted an article on how to get it back into AE CS4.
Here are more informations about the PAR correction.
If it's not your issue, please give us more information on how "smaller" the video is in AE, or try to post a screenshot of your video inside AE so we can investigate further.
If the footage was 16:9 when shot then you must interpret it as 16:9 in AE. Check the interpretation by selecting the footage in the Project window then going to File>Interpret Footage>Main. Field separation is also checked here. The footage is smaller in AE because AE is interpreting it as normal PAL by default. There is no tag in most codecs that define the PAR. The screenshot you have from FCP is info from the capture and FCP settings but the PAR info isn't added to the .mov file.
Your Composition window has two options for preview, Pixel Aspect Ratio Correction on and off. (check the help files if you can't find the button). With the PAR correction turned off you'll see a better image but it will be slightly distorted. The better image quality is because you are seeing every pixel. With PAR correction turned on the pixels are interpolated so there are some aliasing problems.
You can't set PAR when you export to DV codecs because DV codecs don't have a tag for that info. FCP and Premiere know how you set up the capture so they are pretty good at interpreting footage properly. When you set up your renders all you have to do is pick the appropriate codec. If you're not going to do any more manipulation with the image I suggest that you use the same codec (DV) that was used to capture. This will let FCP playback without rendering. If you're going to do a bunch more manipulation with the footage I'd suggest that you export high quality animation codec quicktime. This is a lossless codec that will give you the best results possible with DV source footage. Animation Codec will also hold up to a bunch of reworking without introducing compression artifacts.
I hope this helps. There's an entire section in the help files on setting up comps and on rendering. I'd a must read for someone that's just getting started.
bingo! it was the interpretation - thanks for the help. Please can you also tell me what the benefits are for using square pixels or not - keeping in mind that the output will be onto DVD and DigibetaSP. Thanks
If you work in square pixels AE will automatically correct any properly interpreted non-square footage. You should always separate fields when working with non-square footage if you intend to move the footage off from it's default position.
If you choose to work in square pixels you should create a render comp using the standard presets to match your footage. It's a better idea than to try and squeeze the footage in the render cue settings.
I hope that helps.