Six weeks is still pretty early to be expecting alot of "plumpness".
Give her another week or two. Or like bjh said and I can certainly attest to, she could be having a singleton or two. Check out the "august litters" thread; I have pics posted of Halle...........she whelped a single 6 3/4 ounce puppy a week and a half ago. There are progressive belly pics of her there.