CubeMX V4.21.0 on MacOS 10.12.5.
4.21.0 was available, so I downloaded it. Opened my current project, checked to see if my pet peeves had been fixed yet - nope. Went for lunch.
Came back from lunch and Cube has 3.7GB of RAM locked down.
If I click between the various tabs, the memory goes up in 20MB increments. Now up to 3.89G.
Simply move the cursor around on the screen, you don't need to click on anything, and it goes up in 100MB increments. Now up to 4.94GB.
Just as a point of reference, Eclipse, with a large project open, in the debugger, using OpenOCD, since mid last week 428MB.
Andrei from The Great White North
(as seen on the Embedded.FM podcast and blog)