Check your manual for changing stretch mode.
Usually plasmas will have a zoom, full, h-fill, just, and pillar box mode, or some mix of these modes.
Just play around with these modes and see which you like best.
Most likely right now you are in h-fill or full mode, which stretches everything out from a 4:3 aspect ratio (standard definition) to 16:9 aspect ratio (the shape of your plasma). This will make everything look fatter.
Be aware that if you use the pillar box setting, you might cause your plasma to burn-in. The pillar box setting will set the image to 4:3, and leave black bars on the sides of the screen. Usually there is a setting to make the black bars gray, so make sure you do this to reduce the likelihood of burn-in.
Plasmas are most prone to burn-in during the first 1000 hours of use. To best prevent burn-in, use one of the stretch modes that uses the entire screen during the first 1000 hours of use. Then if you haven't gotten use to it yet, go ahead and use the pillar box mode.
Good luck.