Scala-JS has been making very fast progress lately, the 0.1 release wasn't that long ago. I'm excited to see where it goes and I think there's room for some very cool integrations with Play.
We've managed to get a hello world app down to about 300kb; most of that is the standard library, and even a several-1000-LOC game like [roll](http://lihaoyi.github.io/roll/) clocks in at about 340kb. Getting this number down is WIP.