If your project isn't properly encapsulated, incremental recompilation will often come close to a full recompile, or at least recompilation of a big chunk of code.

For example, if you delete AbstractFooBase.unusedMethod, sbt will recompile every file everywhere that references a concrete Foo instance.

(In principle it should only need to recompile files which reference unusedMethod, but it's not that smart yet.)