Perhaps the React team didn't themselves promote based on file size, But many people have in medium articles etc. I'm the kind of person who works very hard to keep my page size at a minimum, and there are definitely others like me. What do you think of the idea of splitting React up into several files, so you can only input what functionality you want? Do you think that's possible in Reacts code base? For reference, RxJS 5 does this, so you can import the operator flatMap only if you intend to use it.
React doesn't really have much of a public API surface. In Rx you can give up some operators but React doesn't have a similar concept.
That said we will continue exploring how to drop some weight from React further down the road. This is not a priority right now because some large changes are coming relatively soon to React (work on incremental reconciler) so it would be unwise to focus on size optimization before a big refactoring.
I think that we will be in better position to consider what can be removed or separated after we ship the incremental reconciler. Hopefully this will happen within a year. For now I don't have a good answer to this but I'll try to at least provide a summary of how large different pieces are when creating a high level technical overview of React next month.
