I don't know anything about video.js in particular, but many libraries are huge when served from a CDN because they have to account for every possible use case, as well as every locale, whereas in a local build with tree shaking and modular imports, it can be cut down by quite a lot. Depends on how "legacy" the codebase is, though.

True, the CDN size is 158KB gzipped, but it's still under 600KB unpacked.

The point is that's for a feature packed video lib which also covers resolution handling

(not sure if it handles fullscreen resolution changing, but I don't see why they wouldn't.)