Well, generally it gets bigger over time if you apply yourself. It also can tend to mask incompetent play if you can stack it high enough. I would say that a fully heroic geared player could compensate for a ton of bad on timeless isles, but then unless they paid for the runs that got them the gear just the matter of them having it kind of negates any theory that they were bad in the first place. So what is your point?
That I as an under geared raid tank over gear current content?