Assuming there is no physical issues, she could just be skinny. Im sure there are others that can provide some tips. I had a cat that was skinny....nothing was wrong with her....healthy as can be but no matter what I fed her, no matter what the vet told me to try, she never managed to get past 9lbs. She was a petite little thing and she lived a long and healthly life. She eventually passed away at the age of 15 from kidney failure....RIP Zoey
By the way.....Tinker is adorable.
