Java is leaner than C# and C++.
> How can a language and its core library stand still
I guess by having a more abstract foundation and relying less on adding hacks like colored function? Haskell seems to be like that.
Another thing is language extensions that you have to turn on to use. This makes deprecating stuff easier in the future, so languages can trim itself instead of keep growing.
Java looks more leaner, except when really wants to master it, also needs to understand JVM APIs for low level coding (invokedynamic and friends), annotation processors, JVM agents, just like C and C++ how the many existing implementations behave, bytecode rewriting libraries,....
Wider question: How can a language and its core library stand still? To me, standing still is death for any computer programming language ecosystem. Many languages are just getting started on the idea of "green threads" and "colours" (sync vs async). Some of this can be done purely with a core library using existing language features, but some evolutions are better done with language features.