I would think you are feeding enough....have you talked to your vet? Hunter gets 1/4 each morning and night as well as a few small treats (some training some not). He is 12 pounds and needs to go down to 10 for his knees as well. We were feeding more than 1/4 cup and were told to go down to 1/4 cup. I think asking your vest is the best thing you could do as they can monitor Bessa.
Also, increasing exercise is a great way to help drop a little weight off your pooch