Izzy's the same way - she is almost 8 mos and still 2 pounds 5 ounces and she is spayed and we all thought she would gain some from that but not....she gets fed 2x day (1/4 cup) and after a half hour I pick that up. I was not even giving her any people food but then I thought she needed something to get some weight on....I started to give her only healthy food treats like Yorkie Yummies, frozen peas, veggies, etc., so I'll see if that puts a few ounces on her.
But I have been watching her eating patterns these past two weeks and some days she eats a lot and some days not...so maybe they just know when they are hungry and eat.....she is healthy and plays and runs with our 3 cats so she must be OK.