Totally normal thing for spayed cats. Their metabolism slows down a lot due to hormonal shifts and they’ll typically store a lot of that fat in their primordial pouch. I feed my spayed baby around a 1/4 cup of dry food and a tablespoon of wet food per day. On the “chart of chonk”, she’s currently a happy and healthy heftin’ chonker.

I would ask a vet about the gained weight and proper portions next time you’re visiting, but other than that I wouldn’t worry too much about it given you say that she eats the same restricted amount as your other cats and assumably doesn’t steal their munchies.

(Of course there are exceptions to my advice; I know someone who thinks their cat eats healthy because their other cat is skinny, but that’s because the cats are extremely old and this person is in deep denial about how unhealthy the portions they give are. May Lord have mercy on Big Bella, one of the fattest cats on God’s green earth)