I've got a model I've been working on for a few weeks here. The file itself is 2.1Mb and there are no textures used. Although the full model is 3 million polygons, it's all subdivision and mirroring and arrays and most of it is hidden most of the time. I suspect Blender is doing some intelligent lazy generation given there's a slight delay when I unhide objects for the first time in a session.
However, I'm seeing memory usages in this scene at a bare minimum of 850Mb. When I do a render, that shoots up to 1500-1700mb and stays there (doesn't increase on subsequent renders) until I restart Blender. It's causing some minor performance issues when I've got other stuff running and in some cases I have to restart Blender after doing a render just to reclaim the RAM.
I suspect I may have a fault in my objects somewhere, and on using the space search, found memory statistics. Looking it up, it apparently ends up in the console. I could find this on previous versions of Blender by searching for "console" on Windows or by running from a command line. However, on Mac OS X, "console" returns no results and running from a command line shows nothing in the terminal window besides me starting the .app.