Any fandom will have some bad apples in it, it is human nautre and it is unavoidable. But how many people does it take to ruin it for everybody? I think that no matter the size you will find that the number of people stays in proportion. And I would say that that number hovers arouns 5% with the rest of the people being various levels of normal. This does not change with the size of the fandom, yet, the number as you will see plays a role in how they can make a fandom overall "awful" in many aspects.
So let's run the numbers.
Your fandom, this is hypotheitcal, has a hundred fans, when we use the 5% rule that means that you have five people who are going to be complete pricks to everyone they interact with
But the thing is, five people are pretty manageable as a whole. When your community contains 95 other "normal" people they can disengage from the other trouble makers. They can be left them out of events or whatever else you may have planed.
But let's add a zero to the number, you now have one thousand fans.
Taking the 5% rule that we laid out above, you now fifty people are going to be nightmares for everybody else in the community.
What makes the difference here is that fifty is a big number. That is half of the community that you had just a moment ago, and while it may not seem like much, they can easily sustain a message board with that number of userers.
But that is one niche message board that you have to worry about, it is still a small enough number that one board that only these "fans" and nobody else will ever visit. Like this the problem can be contained and they will jst be known as trouble to the cmmunity at large.
Big Numbers Mean Big Trouble!
You have surpassed all of this and have made a really successful property that has gained lots of fans. You don't have hundreds, or thousands, you have one hundred thousand fans yet you are dreading that nasty that 5% problem.
The trolls have grown and you now have to deal with five thousand unrepentant assholes. That is a lot of ass in that hole and you need to see it as something besides a self-contained problem feeding off of itself.
With five thousand people you will keep seeing them re-surfacing in all of the popular fan communities, spreading their spite and making every effort to control the popular oppinion. And with that many people you will not be able to manage their behaviour, isolate them, or shield your true fans.
This number is a problem and with that many trouble makers you can expect that a certain number will visit your convention venue, in fact that many people could completely fill a small convention.
For most people this is not valid though, becuase being a fan requires active participation. And most people will never fill this role. People who felt that they were fans at one point will drop out of active participation, leaving the fandom and with the dropsouts you will also loose some of the toxic fans, but not many. The drop off of normal fans to toxic cans will not be proportional.
A loss of fans can happen because the TV show went off the air, or because the toxic fan base drove people away.
That is what has happened to Rick and Morty. Don't get me wrong, the populararility of the show is huge, but you will notice that toxic fans have begun to take up more space.
The show's normal fans have begun start to leave the central fandom and in essence have only left the worst viewers to dictate how the show should progress. What's left are the obnoxious assholes who throw tantrums over shitty nugget sauce.
It's a distillation process, as the more "dedicated" fans are the ones who stick around and continue to identify with the work. These more dedicated fans are probably more likely to be toxic than the average viewer, in a lot of ways.
Steven Universe is a network-produced television show with something like two million regular viewers. Even if only a tenth of those people would identify as fans, that's a staggering number. Even a small proportion of toxic fans would constitute a large toxic community.