In response to the unnatural edits:

Sunny has a nice round butt on her longer body, big as G4 but without the spine sharply jutting upwards. More like an actual horse. The butt also connects to thighs like a real horse, which G4 anatomy minimizes.

As for the lack of butt crease, a theory for why this is: it’d look too lewd. Sunny’s almost peach-colored coat would make a butt crack look lewd because the 3D allows for realistic shading and lighting, which gives it depth. G5 has also shown there’s jiggle physics for their butts! Now they can have the ponies from any angle or pose they please without concern of it looking suggestive because of one detail. These aren’t one-off horses, it’ll be ponies on screen every second. Note the film was made for the big screen before being pushed to Netflix.

Even in G4, the crease goes away when you look at the ponies directly from behind.

Pipp also has a bigger rear than any of the mane ponies.