
If she is eating so little I would keep her on the high density (puppy food). Unfortunately some dogs have this tendency to only eatr just enough to keep body and soul alive.
I have a 5 year old bitch that is like this. I call her the Super model. The only time her appetite was really good was after she had her first litter of puppies at age two, she then ate beautifully and was in lovely condition (apart from the hair loss normal after rearing a litter) throughout lactation and until her next season when pups were 5 months.
After that she was the same as before, but maybe not quite as bad. Everyone thinks she is a puppy, and her daughter is a big beefy girl who could eat for England given half the chance.
I have found the only thing that really works is to give her food twice a day, and leave it for 15 minutes and take it away after.
Do not on a good day give her more food when she has eaten her portion, as then she won't be hungry next meal time. Took me ages to work that opne out. My bitch used to eat 3 days out of 7, so when she ate her food I would top up her dish, and was really pleased if she cleared it, only to find she went on hunger strike next day!!!
The plus side of this is she is likely to have minimal problems with her bone growth, as he won't be overloading the joints. You will probably find that her muscle will be hard and wiry too, and as long as her coat shines, eyes sparkle, you will just have to largely ignore her lack of body. The other plus point is that she will retain a youthful figure into her middle years.