Your timeline resolution is most likely not set to full. This renders the video preview at less than 100% for performance purposes. If you set the timeline resolution to Full, scrubbing through the timeline may lag depending on your file and your computer hardware.
You can read more about the resolutions options on Adobe's help page
Auto: (available only for previews) Adapts the resolution of the view in the Composition panel to render only the pixels necessary to preview the composition at the current zoom level. For example, if the view is zoomed out to 25%, then the resolution automatically adapts to a value of 1/4—shown as (Quarter)—as if you had manually chosen Quarter. If a panel contains multiple views, the resolution adapts to the view with the highest zoom level. This setting gives the best image quality while also avoiding rendering pixels unnecessary for the current zoom level.
Full: Renders each pixel in a composition. This setting gives the best image quality, but takes the longest to render.
Half: Renders one-quarter of the pixels contained in the full-resolution image—half the columns and half the rows.
Third: Renders one-ninth of the pixels contained in the full-resolution image.
Quarter: Renders one-sixteenth of the pixels contained in the full-resolution image.
Custom: Renders the image at the horizontal and vertical resolutions that you specify.