Why Does X Reduce the Quality of Videos That Look Sharp Before Upload?

I’m trying to work out whether this is an export issue or something X does during upload. My clips look crisp in the camera roll and in the editing app, but the version shown in the feed often looks softer, especially around text and fast movement.

Does X apply different video compression based on resolution, bitrate, or file size? I’d be interested in what settings people use to keep detail without making the file unnecessarily large.