It's easy to see that most people just aren't into the lower weight classes, particularly as regards flyweight. I'm guessing most fans have some sort of limit to their interest in smaller fighters that seems to start kicking in around lightweight. Like there's some issue of materiality where fighters of a certain size just don't matter to you as much. But the time you get to 125, most people are under the threshhold of where the fights seem relevant or interesting to them.
In fact, behind all the sloppy throwing around of terms like manlets and all the over the top lionization of flyweights in response, there seem to be some interesting psychological principles about how someone's physical appearance or size affects how we relate to them.
Personally, I respect flyweight fighters, but I admit it takes more effort for me to care about that division than welterweight. That said, it's easy to understand why someone without the same limited interest would find the fights entertaining: there are several very skilled fighters at the top of the division and the fighters are fast and have cardio for days.
