I thought that a while ago I had read about Scala using "special" implementations when creating collections for a small amount of elements. E.g. for a
def checkCoordinates(x: Int, y: Int, z: Int) = Seq(x, y, z).forall(_ >= 0)
Sets are optimized you can see here:
And same approach was taken for maps
List it seems like a linked list (::) is created no matter what (not optimized).
Keep in mind you shouldn't really worry about this kind of things until profiling shows you that is causes performace issues for your application.