If I remember correctly, for 2.12 the compiler will generate additional forwarder methods, which mitigates the performance issues, but increases file size again.
It's still a lot smaller than 2.11, but for instance the standard library of 2.12.0-RC1 was 900kB smaller than 2.12.0.
[1] http://www.scala-lang.org/blog/2016/07/08/trait-method-perfo...
[2] https://bugs.openjdk.java.net/browse/JDK-8161334