On bootstrap, Foundation, semantic UI you have all of the CSS code included, on tailwind and windicss you only include what's actually on the page by compiling it.
To a certain degree and also on semantic, but it always have a core CSS which is largish, windi and tailwind is on a granular/utility basis. So it's often something like 8kb for a page with tailwind/windi but +100kb for semantic.
https://medium.com/@jacobp100/tree-shaking-bootstrap-95d6301...