By the definitions you offered I guess I'd fall to the introvert side. My mate would definitely fall to the extrovert side.
The problem I have with large groups is not with a 'comfort' level but with an inability to concentrate with a big volume of background noise/activity. And maybe that's where the 'draining' component comes in - trying to absorb all the stuff from multiple directions.
I'm the type that attempts to be fully present in a conversation, trying to get the full range of expressed communication. Tone, body language, hidden (or fuzzy) meanings etc. Too many distractions make this difficult. Add to that, if I pick up something interesting going on in another conversation that peaks my interest, I want 'in' on that (focus). Now I'm caught between being rude (not present in the 1st conversation) and pulled away from what may be a more interesting or relative conversation. So my poor brain starts oscillating -bing/bong, bing/bong. Net result - I end up losing much/most from BOTH ! I've always questioned that humans can TRULY multi-task with anything that requires or deserves real focus. Not convinced it's possible.
Now of course I'm not sure how much this applies or matters when you're only talking about frivolous conversation/interaction. Meaningless small talk. In that setting it doesn't matter. But I'm not big on that kind of environment anyway - seems mostly a waste of time to me. Comfortable ? Sure. Preferable ? No.
Her - she's fine with that. Passes time, provides entertainment value etc.
Each to their own - right ?