This is the single most common disappointment with any Bluesky downloader, and it has nothing to do with the downloader.
What Bluesky keeps
When someone uploads a video, Bluesky re-encodes it and publishes two versions: one at 360p and one at 720p. Those are the only two anyone can fetch.
The file the poster started with — which may well have been 1080p or 4K — is not served to the public at all. It is not hidden behind a setting or an account; it simply is not offered.
Why no tool can beat it
Every downloader, including this one, can only fetch what the platform publishes. If a site claims to deliver 4K from a Bluesky link, it is either upscaling 720p and calling it 4K, or describing a capability it does not have.
Upscaling adds pixels but no detail. The result is a bigger file that looks the same or slightly worse.
Which of the two to pick
Take 720p unless you have a reason not to. It is the better picture and the files are small by modern standards — a minute of video is usually in the low tens of megabytes.
360p is worth choosing on a metered connection, or when you only need the content rather than the quality.