My cats are both fully grown and naturally small so yes, it's possible. Sophie's about 6 pounds and Lilith is downright tiny at just under 5 pounds. They came from a backyard breeder and I think they bred them to be small. The breeder was either evicted or foreclosed on and left all of the cats behind at the house. They were found and adopted out.
It really depends on the cat. Each one is different. Also, cats really don't stop growing until they're about 18 months old. Here's a body condition chart you can use to determine if your kitty is at a good weight. If you're concerned, discuss it with your vet.
Do you have pictures? That's the only way to tell. My cats are 12lbs but not fat, just longhair and have very large bones, they were 4lbs at 12 weeks of age lol and I think all 3 of my cats must have some maine coone somewhere down the generations. My 7 month old kitten is almost 9lbs lol. Not fat, just long. My boys dad was a 14lb shorthair tomcat, pure muscle no fat. Check out this chart to figure out where your cat is on the body scale.