1. It's not always single tab. JS heap may need to be shared between multiple tabs due to sharing of objects via `window.opener` and other leaky cross-frame hacks.

2. The process will sooner run out of addresses in the 3GB space rather than actual memory. After running for a while a process may end up with objects allocated all over the address space, so that there isn't enough contiguous address space free for new allocations, even though there's still plenty of free memory in small chunks between live objects.

Wouldn't surprise me if Chrome defragments individual tab heaps

The only thing I can think of is something that's doing heavy WebGL rendering, or some sort of js memory leak.

Also a tab that is playing a short low resolution clip using Flash Player.