On my Mac OS X 10.10.5 system, here are the results of calling
MemoryInUse immediately after startup (fresh front end, fresh kernel) on various versions of Mathematica:
As you can see, Mathematica 11.1 uses up a lot more memory than the previous version (I wonder if that explains why it feels much less responsive and takes much longer to load than earlier versions).
Can you reproduce this on your system? Does anyone know why version 11.1 in particular uses so much memory?