The behavior is normal and the whole point of doing stuff for video. On a computer, which is a square pixel device, video resolutions will look squeezed, because the computer has no knowledge of them actually being rectangular. however, in AE you can use pixel aspect ratio correction in the comp viewer to compensate. In any case - do not simply change your comp resolution. The proper way to treat your existing composition is to nest it into a new composition and have AE do the pixel conversion. If you only change the comp, positions may get out of whack or some effects will look different, which is not what you'd want.

Mylenium