This topic comes up quite frequently with my private clients. Dogs of all ages and sizes can be finicky eaters. While I normally don't like to contribute to "breedism" I have found in my experience that small breeds can be more finicky than large breeds.
Dog Training Blog
Dog training tips by Jeff Millman of Chicago Paws.
When clients report that their dog is going potty numerous times per day and they are having house training problems, I always recommend that they look at the quality of the food.