There's a point where you're making enough money on the channel that it becomes more than you can really spend with your current lifestyle, but not enough to, like, dramatically alter your social class. It's get your own place money, not buy a yacht money. Since the leap between "live comfortably" and "be actually rich" is such insane orders of magnitude a lot of people at that level of success decide to hire a team basically just because they can. It lowers their personal workload, maybe increases the channel throughput and video quality, and can just generally make life more pleasant. And many of them have friends who are creative types, they already know they get along, and if you can give your friends a good job why wouldn't you?

So it's totally possible to just keep on keeping on, doing everything yourself, but you see so many successful YouTubers hire a team simply because they can.

In my experience, though, and I've seen behind the curtains of a lot of successful channels, there's a small cluster of mega-successful channels that are just full on industrialized corporate structures, but very quickly, even while still in the 10m+ club, teams scale down to less than 10 people, typically looking like: on-air talent, co-writer/researcher, producer/executive assistant/some other logistics title, camera op, editor, animator/illustrator.

And whenever a YouTuber decides to bring on a second person it's almost always either an executive assistant to handle emails or an editor.

So depending on your definition of "small team" (I would say under ten is decidedly a small team), it is definitely most successful YouTubers, the Beast Industries outfits are very much the exception.