I'm surprised I haven't read the real answer/problem here although there is no solution. I see you mentioned you don't even have to open a project but generally the problem occurs when using storyboards. I've confirmed this, sent examples to Apple, filed bug reports, waited for new releases, nothing yet. Somewhere down the line there is a leak when editing storyboard views. If you are editing raw code you will never see the ram usage increase but the second you begin editing a storyboard with multiple views and segues the problem arises. I currently run 24GB of ram and it can be consumed in less than an hour when editing views in a storyboard. Like others have mentioned, the only solution is to restart Xcode. Simply closing the project is not good enough.
