Indeed, once you end up pulling in most common Scala classes (collections, futures, etc.), adding new functionality to your app increases app size by very little. Our app has ~600KB gzipped js (attributable to Scala.js compiled code) + ~200KB of React and other libraries. The overall size has barely moved beyond +-100KB in last few years despite adding new functionality all the time.

Once Scala.js supports lazy loading of modules (hopefully soon after 1.0), the size issue will have significantly less practical impact.