Oh_First Experiment with After Effects

Since it was the first time to use After Effects, I wanted to get used to the software and to explore its features. The experiment I did is a 10-second animation of a cat that is originally from a Photoshop file containing multiple layers. Each layer has a cat's different body segment, and I played with them by rotating, controlling scales, applying effects and changing positions. After I am done with playing and rendered, I expected to get the cat's body segments flying around in a 1920x1080 screen. However, there is a smaller square border on the center of the screen, and the segments just fly inside the square. Maybe I did something wrong when I was experimenting. Through the experiment, I learned how to play around with Photoshop layers by After Effects, but I think that I need to work on compositions or screens not to get another mistake... Also, after I figure out to fix the mistake, I want to work with not only a single Photoshop file but also multiple files or other Illustration files.