Randall noticed a 6GB gmail tab, he was able to repro with a 2.6GB tab.  He checked JS heap usage with devtools, and that was less than 150MB, so we must be leaking something in the renderer.

I originally opened http://b/122262118 for the gmail team, but now it seems that it's a Chrome problem, so I am closing that one.  Please find more details there, including a feedback report and a screenshot with "top" showing memory usage in the renderers.