Worth noting is that Immutable greatly benefits from gzipping, so the actual size comes down to 15.21 kb (with level 9 compression).
That said I definitely think it would make sense to provide a build preserving ES modules for tree-shaking purposes (and/or the top-level API as individual CJS modules we could import selectively).

Yeah this would be great to have, but it's not easy to implement incrementally - some breaking changes would be necessary.

Specifically right now all types have a toList() and toMap() etc methods which convert between various collection types. That ties all of the library together and tree shaking typically isn't smart enough to trim prototype methods as well. A breaking major version change would be necessary to remove these methods to enable tree shaking.

However I don't actually think you would shave much from the ~15kb build already. That's roughly split into thirds: Map, List, and everything else.

Merging into #317

I have just one note regarding toList() and toMap() methods on all types.

Maybe RxJS operators are similar case: import 'rxjs/add/operator/switchMap' adds switchMap operator to Observable interface in ES import/export friendly way.