I have a 1280x720 square Composition
I have brought in a 1280x720 .png (and other 1280x720 .psd files) when I drag or add them to an existing Composition they come into the Canvas like this
They should of course fit to the size of the Composition the only way I can get them to is to Shift+/ but the image then goes to 170%
I have trashes prefs and that didn't sort things out and I have also opened a new project and tried the same thing and it still is happening.
AE CC 184.108.40.206
You are confusing Zoom Scale Factor with composition size. The Zoom Scale factor us just the scale factor of the composition window and has nothing at all to do with the image. When your Zoom Scale factor is 100% then you are seeing a pixel for a pixel. IOW every pixel in the image is represented on the screen. Set that box to 800% and you'll more than fill the viewer and the image will look pixilated, but this has no effect on how the project will render. You just get a closer look at the pixels.
I'd suggest that you start with the basics. There is a link on the home page of this forum.
A screenshot that showed us the timeline and the comp window along with the composition settings would be more helpful. It looks to me like your monitor is very high resolution or your composition is not setup with a standard 720P comp size of 1280 X 720 pixels