Rather than extroversion being about whether a person is socially "energized" by groups or not, it's really about how much stimulus a person can accept before their brain starts feeling saturated. IE, it's less about the social aspect and more about being able to process stimuli in general. It's just that social gatherings tend to provide a lot more stimuli (people are complicated!)
In this model, an introverted person is simply a person that becomes saturated much more easily; and the reason they dislike crowds and other large gatherings is just because the amount of possible inputs are tremendous and need to be kept track of. An extroverted person might feel more comfortable in these situations, either because they can process more stimuli, or more likely in my opinion, because they're less sensitive to those stimuli. (I don't think it's a coincidence that a lot of the introverted people I know tend to pick up on subtle social cues a lot more readily than extroverts, and also tend to emit more subtle cues).
As a person that's probably moderately introverted, thinking of it in terms of "saturation" feels more natural to me. If I'm in a large group I don't so much feel a loss of energy (unless I'm trying hard to be extra friendly); if anything, when I reach a saturation point I'm at a higher energy level, I'm just sort of "full".
I wish I could remember where I first read about this way of thinking about it though and give some sources.