It's actually super light, i don't know where you got the impression that it's heavy. This is not a monolith but a modular kit - you only load what you use which makes it different from many other UI kits which give you megabytes of CSS on first load.
Also, this is native HTML and CSS of course, with proper definition, types and handlers laying out what the component can do:
To "just" use self contained, configurable, property-defined components that can be shared, re-used, filled with state - that isn't trivial. Which is why they're React components. The components themselves are very lean, there's a bit of code, some markup, a piece of CSS and that's that. The code i have linked compiles down to a couple of bytes.
Also, this is native HTML and CSS of course, with proper definition, types and handlers laying out what the component can do:
https://github.com/ant-design/ant-design/blob/master/compone...
https://github.com/ant-design/ant-design/blob/master/compone...
Resulting in: https://ant.design/components/button/