That's just nonsense, maybe 6 or 7 years ago that could have been the case. There's an approximate 100KB "tax" for pulling in Scala collections (i.e. as soon as you depend on List, Map, Seq, etc.), which is the price to pay for being able to use the power of Scala in the browser.
> Is there any way Scala.js can produce code as compact as BuckleScript/ReasonML in the future?
No, iirc, not without a redeisgn.
There's a longstanding issue wrt lazy loading, which is slated to be worked on at some point post-1.0 -- I'd say that's probably the biggest missing feature, the ability to break out one's app into dynamic chunks, so for example a lightweight frontend/public facing area of an app can be isolated from the backend where all the heavy lifting is done.
Perhaps with Dotty (AKA Scala 3) next year there will be further reductions in Scala.js code size, but no, nothing like BuckleScript.