I looked up this info on Shih Tzus:

Size-Ideally, height at withers is 9 to 10 1/2 inches; but, not less than 8 inches nor more than 11 inches. Ideally, weight of mature dogs, 9 to 16 pounds.

So if she's around that height her weight is fine, on the high end even.

I'd only be concerned if her weight dropped and she was lethargic. Not all dogs eat everything in sight as I learned with my current dog. Some days she will skip meals or hardly eat, don't know why. For 9 years her weight has been steady - 35 - 38 lbs - it's as though she knows exactly how much her body needs to maintain that weight. I know a lot of people who would love to have that ability and discipline! (Of course Sadie doesn't do it for her figure.) Her teeth are good, coat is shiny- never had a health problem. Picky eating could be a good thing.