Jewel has always been super skinny - probably a function of the fact that she runs and fetches constantly. To be honest, we have been jealous of all the meaty Shih tzus we see running around. To combat this, we started feeding the dogs raw meet.
The mix consisted of buffalo meet, vegetables, and pepporoni flavoring of some sort. Jewel likes the stuff, but Willow loves it. Jewel isn't that into food. I think because she doesn't want to be forced to do tricks. Willow knows that Jewel moves slow towards it, and wont defend it, so the trick is to keep Willow from inhaling her portion then Jewel's. I learned this because I feed Jewel a larger portion and Willow got in, sucked it down, and got out before I could react.
Having learned to give Willow her portion first, Jewel is able to enjoy hers. As a result, Jewel has gained weight. You can feel it when you pick her up. Mommy thinks she looks more beautiful as well.