I know what you mean. I should be proud of the weight that I've lost, but I'm embarrassed about how much I weigh, thus how much I have to lose. Yep, it's something I need to work on.

I usually say something like "Less than I need to and more than I care to admit". It seems to lighten the mood and the point is taken that it's not something I want to be a topic of conversation. However, if the person that I'm talking to is overweight or seems sincerely interested, I talk about what I'm doing and how successful it has been for me. My hope is that it will encourage them to lose weight, get healthy, and feel better.