There are definitely differences here and there. One I can remember from a year or so ago is that Preact deprecated support for string refs while React still supports them (though the docs point out they intend to deprecate IIRC).
Honestly to my mind the biggest thing is that bundle size just isn’t a priority for the React team. A sibling comment pointed out that React does a lot to correctly handle SVGs. Good to have, obviously, but what percentage of React users are making interactive SVGs? Probably not a whole lot. If they wanted to the React folks could break that out into a react-svg module or something and save some KB. But they don’t want to.
To my mind React these days is the enterprise software of the frontend. It’s big, it’s bulky, but it does absolutely everything with a minimum of fuss. They have a one size fits all approach and they’re not willing to deviate from it.