Maybe people avoid average body types because it seems too human-like? Or maybe the average human shape doesn't meld well with the basic frog shape?
As a newt, I'm slender cuz it's common for terrestrial-stage newts to have that physique.
Obviously the design of a character is based on the desires of the creator or owner, but I also think frogs (and some toads) naturally fit into that either/or kinda situation. They're either highly active and mobile species which get adopted as muscular/athletic anthros, or larger and slower species which are adopted as heavy-set ones.
I love Pacman frogs, and despite the fact they are typically really muscular, they are also very round. and I think it would look weird to depict one as lithe or skinny. On the other side, Red-Eyed Treefrogs are super skinny and active and it would seem strange to see one as fat or bloated.
I guess Bullfrogs could offer options though, as they could easily be shown as heavy either with muscles or with fat. But then again, a skinny Bullfrog would still look weird.

Thanks for sharing this, I totally understand what you're saying about the link between the creators size as I've met quite a few of the froggy members and those with slim characters tend to be small-framed and visa versa, interesting!

Ranito and his pudgy belly weight at 110 KG / 242 lb because that's my actual weight. That's right fellow sexy frogmen! The human me is as handsome as my frog self!

Joking aside, apart from representing me my fursona's fatness and gluttony are an integral part of his personality in my stories. Both for comedic and fetish-y purposes

I get ya! Big personality/body=Big Froggy!

Mine is 5'2.5" and weighs around 216 because that's what im like IRL.

I try to keep my fursonas similar to me...!

He's chubby but not too big.

*nods* great to hear, it seems to be that way that frogs/toads match their creators more than I thought, always good to be surprised!

Chwii's a hybrid froggeh, but really the reason I made him have a big tum is that I heavily took off of what his two inspiring species are built like... He's part Desert Rain Frog and part Gastric-Brooding Frog, but largely Rain frog. The Gastric-Brooding Frog parts just made him taller and more water-loving. ^^

But true, it is rare to see frog or toad characters with different body types than slender and treefrog-like or big and round like a pacman frog. XD Maybe it comes from those two types of frog being what most people think of when they hear "frog"?

Very true, I either see frogs/toads as thin/slender or big and tubby - personally my leanings are towards the big and chunky as theres more to cuddle and I just love curves, but I adore many with slender and more athletic forms as they tend to be more colourful in design :D