Hi and welcome to YT and congrats on your new baby! Just wanted to let you know that there is no "miniature" yorkie - a yorkie is a toy breed that should be 7lbs and under, but you'll see them up to 20lbs. There is quite a variety in sizes here on YT

Your baby seems to be close to my Lacy's size. Lacy is about 7.5lbs (3.4kg) and her chest size is 14.5in (37cm). So your yorkie is probably close to its ideal weight. To be at an ideal weight you should be able to easily feel the ribs underneath skin and a thin layer of muscle/fat, but you shouldn't be able to just see them when looking at the dog.
I have Lacy on a raw diet, but there are a lot of members on here that homecook for their yorkies so hopefully they can give you some good recipes to try. There is a search button at the top where you can search for Homecooking and hopefully you'll get some good threads to read. I do use a fish oil/vitamin E. supplement with her food as well as Juice Plus for dogs.
We don't really go on long walks normally - just short ones to go potty. She seems to get most of her exercise inside playing.
I bathe her once a week and brush her 2-3 times a day. I have her nails trimmed about once a month and trim up her hair a little about once a month too.