In addition to what Micheal suggests (not in place of), it’s important to make sure that the aspect ratio and dimensions settings for both your comp and the layers within it are what you want them to be and that you’re viewing everything correctly.
For the first part, go into the comp settings and make sure the aspect ratio and dimensions are correct. Also, (as Micheal pointed out) check the same things in the layer settings.
For the second part, make sure you are aware of your comp viewer’s aspect ratio correction setting (there’s a toggle button at the bottom of the comp viewer) and that the “correction” feature only affects what you see while working (not the actual output).