X compresses hard, and the numbers explain why
X caps most uploads at 512MB and re-encodes everything to a modest bitrate, typically 1080p at around 5 Mbps for the best case and far less for short clips. The file you download is the file X serves — there is no hidden higher-quality master sitting behind the player.
GIFs on X are not GIFs at all. They are silent looping MP4 files, which is excellent news: you get a far smaller, far cleaner file than a real GIF would ever be, and it can be converted afterwards if you genuinely need the old format.