Just think about it, you and a couple of friends are watching classic South Park or something edgy. And then, some person comes who's not even a fan of it, demands changes (censorship) or outright turns off the program. Just why do people do this? Why do they ruin everyone's fun?
Seriously, they are. This is by far the most important issue, if they made their asses fatter it would improve viewership. If they made the ponies thicc prudes would complain, which would only draw in even more viewers. Twilight Sparkle, the protagonist of the series has a fat ass, and her teacher Princess Celestia, who's ass is even fatter.
The main reason is uncanny valley. When something looks human but not quite. A perfect example would be a robot trying to mimic being human, it looks unsettling. It's somewhat insulting towards the ponies, you have Twilight Sparkle there a beautiful unicorn and you can't embrace her just because she's not fully in human shape, even though she has all the qualifications to be a sapient-minded
Baby RD, the most adorable thing ever.
Pinkie!
Applejack.
AppleJack.
Applejack
Yes.
Princess Luna for me.
3142754
ditto.
And do you want to give them all a big, inappropriate hug?
3142780
... Yes...
High five!
fluffle puff chubby
3142825
Indeed.
Twilight is best pone!
3142589
Tis so, obvious?
Fluttershy
Giant Celestia of course
3143855
You could probably hug a small part of her face when has her head leaned down.