We've had our Keurig about a month or so. Ours gives us three size choices - 6oz, 8oz, and 10oz. We tend to use the 8oz most of the time. I use the 6 oz too, and my husband will use the 10oz.
According to the manual, I know the resulting amounts are actually a little less than stated, but I tested this morning with a glass measuring cup, and got these results, which track with our experience:
6oz - OK
8 oz - closer to 6.5
10oz - less than 8oz
Do we have a defective machine or is this your reality too? We use distilled water in ours to avoid mineral build up.