I would guess unlikely. The reduction in JAR size as mentioned there is due to relying on Java 8 now which gives more JVM-native approaches to things Scala was

I would guess unlikely. The reduction in JAR size as mentioned there is due to relying on Java 8 now which gives more JVM-native approaches to things Scala was otherwise emulating (e.g. multiple inheritance). Scala.JS works off of an IR and is not related to native JVM constructs or JVM bytecode.