I went through this exact thing with Uni. She was really skinny and everyone who saw her would comment. She was still active and healthy though. For a 5 lb yorkie gaining or losing a full pound is 20% of their body weight! So before I found YT I went on yahoo groups asking and they were no help, saying one pound to gain is no big deal
But you ARE supposed to easily be able to feel ribs and see a waist from behind or sideways when they are wet during a bath is easy to tell.
Now let me ask you, is your baby spayed? After Uni got spayed it was like someone turned on her hunger switch, and now it is SO easy to feed her, in fact she lets me know when she's hungry, like a dog should!
I feed her the same food before and after her spay, and her appetite is much better now. But now I have to make sure she doesn't overeat. LOL
I have to say Cesar wet food is much worse than Royal Canin. It's equivalent to junk food. I feed Honest Kitchen, it's a dehydrated raw food with human grade ingredients, nothing you can't pronounce. It's got the consistency of oatmeal after you rehydrate it, and they have several formulas. They sell sample 1oz packs for $1 which is about a full days worth of food for one yorkie. Check them out here:
http://www.honestkitchen.com
Also check out other types of dog foods here by rating. Check out Cesar, Science Diet (listed under Hill's), Kibbles and bits, just for fun, or perhaps not so fun.
http://www.dogfoodadvisor.com