It's not the pound weight that matters in terms of the crate size.
(Total crate+cat weight does figure into the price at time of weigh-in, but that's another matter).
Your cat could be of small height and small length but be 25lbs (heaven forbid what kind of little round creature that would be) and still be permitted to be in a smaller crate.
What they want is a certain specified CLEARANCE on the actual size of the cat in terms of how they fit in the space and how they move within it.
They're looking for how much room your cat has, clearance is the key word. Not the weight of the cat.
The information I was given by a pet export specialist, namely a woman called Sandy Lehrack at VIP Pet Transport, is that the when the animal is standing still at full height, there must be at least two inches, ideally three inches, between the top of his ears and the inside "roof" of the crate.
In addition, he must be able to stand up and turn around without any part of his body touching the sides, front or back inside the crate. Presumably the tail brushing against a wall is okay. It's the body that must have room to turn completely around without a tight squeeze.
The animal must also be able to lie down fully.
What it all boils down to is that even a small or average sized cat usually does wind up in what Petco deems a "dog" crate. All that matters is the clearance inside. They've got to have those inches above the ears when standing erect.
Having measured my cat as best I could while he was standing around, my crate too wound up being the 27.5 x 21.5 x 21 inches crate. He only weighs about 11 or 12 lbs.
They want the animal to have plenty of room to stand fully if he wants to, and turn around fully when he needs too, rather than be crouched into one positions, because he's going to be stuck inside there for usually a long flight. They want it to be much roomier than the tiny crates we can cramp them into for a quick ride to the vet's.