I have the same issue than you, I find my dog too thin, but friends who have also had puppy yorkshire tell me that his weight is normal
Just read lot of article for hours for find "the best food", but for exemple I found
The Best Dog Food For Yorkies – Keeping Your Dog Healthy | Wealth For My Health, so yes it's an interesting article about what vitamin, amino acid or something in each food but to be honest I really don't know well about nutrition in general and I want the best for my love, I'm will be really happy if someone can tell me which is the best dog food, but of course natural food not with chemical products!
Thanks in advance