The zipper top opening is smaller than the top or bottom measurements. I keep it buckled in the seat belt in my car, so I can put my 5 lb puppy in through the top. At 3 lbs, she fit easily. At 5 lbs, she has to go in either feet first then I have to wait until she bends her neck sideways to get her head in, or she can go in head first, and I have to tuck her feet in. My Pekingese has a lot of hair, too. If you have a long-haired pet, it can get caught in the zipper.
The seatbelt takes up some room inside the bag because it squishes the bag in. I wish it had a better frame that was stiff. I wish it had a larger mesh breathing area on the seat belt buckle side. That is the smallest area of mesh which is about 3" x 5". My puppy gets very hot inside it.
Now, at 5.5 lbs and 4.5 months old, she has outgrown the zippered opening. I will buy a 100% mesh sided bag with a better frame when I replace it this week. I used this bag only 1.5months, but only for traveling to the vet, so I can't say how long it will last.
I honestly can't see getting an adult cat inside it that doesn't want to go inside it.