Note: it may also be lower res as I have AE set to that to speed up performance when editing.
You need to learn about pixel aspect ratios and how to work with them. My suggestion is that you use the Square Pixel PAL Preset (widescreen or standard depends on your project) and then, if you must, render to the appropriate DV format. The problem with working with non-square pixels is that it is very easy to foul things up with the original artwork and the output, and anything you render for the web will be distorted because the web and mobile and hd video is all square pixels. If you want to continue with your current project you can just change the Composition settings to PAL DV Widescreen Square pixels or your can turn on pixel aspect ration correction preview in the composition window. I would strongly suggest working with the Square Pixel preset.
Personally I would never spend time producing a SD project today. I would just use an HD preset 1920X1080 at the proper frame rate for my audience (29.97 for NTSC countries 25 for PAL). You can always render lower resolution copies with the output module (without adding hardly any extra render time or use the AME to produce your deliverable product using one of the many presets that are included.
Thanks. Switching to the square pixel preset seems to have done the trick with the dimensions. As you can probably tell I'm (not) an expert on AE.