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Chihuahuas tend to be pretty tiny dogs. They do frequently look like they are skin and bones because they are extremely tiny dogs. A 2.5 pound chihuahua would be a teacup variety. These teacup dogs frequently do have health issues which is sometimes why they are so tiny. But if bloodwork is normal and the vet says she is healthy then she likely is. And adult chihuahua only needs about 35-40 calories per pound each day. The food she is on does state that she should have about 3/8 - 1/2 cup of food per day, so she seems to be eating a little less than is normal for an active adult. In addition, if she isn't walked regularly then she may not need as much and it does state that the amount to feed can vary up to 20%. She isn't that far off. If she gets any treats, then that would reduce the amount of dog food as well.
You might try taking her on regular walks to see if that improves her appetite. Sometimes adding a small amount of yogurt to the food helps them eat a little better. Chicken is a bland food and most dogs will eat that, but it is not a balanced meal.
Here is a chart to determine if a dog is underweight or overweight or just right.
Vets see a lot of different dogs of various breeds and are really good at letting you know if your dog is underweight or overweight. I'm pretty sure he would have said something if she was underweight. Chihuahua can tend to be overweight, so being the ideal weight or a couple of ounces below the ideal weight wouldn't be so bad for a chihuahua.
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