My threshold for a 'big' creator is basically this: can you make a post and feel guaranteed to receive commentary feedback (as in, beyond just a like)? And not just from friends or the same 4-5 dedicated followers, but from any one of a pool of at least 20 different fans?
If you can answer yes, I'd consider you to be 'big', simple as that.
It's a much tougher threshold to cross than just having certain numbers of subs or views or whatever...and because of that, it scales to different sites. Like, 50 subs on Webtoons is equivalent to 5 subs on Tapas? 10 Twitter followers is equivalent to 30 Tumblr followers? Great; doesn't matter. This rule stands. ^^
I think it's truly reflective of engagement-- of not just having a fanbase that likes you, but having a fanbase that feels connected to you and wants to do things for you; even if it's something as simple as letting you know your work made them 'lol'.
When you get to that point, that's when you can actually consider doing what you do for a living.