Sometimes it's just a mystery. My female cat has been thin her whole life - eating like a garbage disposal and not gaining an ounce, but staying probably at about 8-9 pounds (but I could feel her spine and ribs, usually). Due to recent issues, she did get a couple CBCs done - one vet suspected hyperthyroidism, but her thyroid was only a little tiny bit overactive. As in not enough to warrant medication for hyper-T. She's a domestic shorthair, so as far as I know, she's not meant to be a large/thick breed. So I have to assume she just needed food that had higher nutritional quality. I started giving her Purina Kitten Chow (kibbles) a couple years ago along with a can of wet food every day (unless I ran out, in which case I would look in the fridge for deli turkey or poultry to feed her). She bulked right up and eventually hit a weight plateau of probably 13 pounds. She was still very long and slender, but kept her weight up. And if she didn't get fed often enough, she could easily drop several pounds. There are times I'd go out of town for 3 weeks and leave the cat in my mother's care, and when I returned home, my cat had easily lost 3-4 pounds. But I feed her normally for a week and she gains it all back. Easy gain, easy lose with my girl. Due to her illness, she managed to lose about 2 pounds in as many weeks, but gained a pound after 2 cans of cat food over the span of 48 hours. Meanwhile, her brother eats normally (and eats special urinary tract health kibbles) and he's at least 20 pounds. It could be plain old metabolism - some humans can eat like hogs and not gain a pound. Meanwhile, others can have a cupcake and gain 5 pounds. Sorry, I'm derailing and going on about my cats. It's good you're getting blood work done and that will tell you if anything is amiss, but also just wanted to ease your nerves and let you know that sometimes a cat is just a little on the small side. Is it possible your kitty may just be the runt of the litter or is just naturally small despite her breed? There are always size anomalies among all species and this can easily extend to cats. I hope you receive good news from the vet, in any case.