I didn't check all of them... I wanna say they range from ~200 to ~500K subscribers? No idea if that's big or not. For comparison, the official Warhammer channel has ~900K subscribers, which I assume is decent.

The argument I've heard repeatedly from them is that the time and effort involved in making a YouTube video that gets enough hits (which means lots of experimentation) is disproportionate compared to the meager return of investment; that for money reasons it's best to get sponsorships.

(I'm not a YouTube author myself, I wouldn't know what's a decent size).