My response to this really depends on to whom I am speaking. If it is a casual but polite acquaintance, I quietly respond something like "more than I might want to admit, but thank you".
If it is someone who appears they might have a similar issue, I will tell them and often they then express interest in IP. The risk of stating the actual number is outweighed by the possibility of being able to help someone.
In general though, I am mortified by how obese I was (hard for me, still, to use that word). I was in incredible denial. I knew I was overweight but unaware of how HUGE.
It worries me that now, even almost 100 pounds less, I am not seeing how I really look. How do I know when to trust what I see and what I am? If I could not see the extra 100 pounds of fat, how can I know whether I am capable of still knowing what my body looks like and what areas I still need to work to improve? How do I know when an outfit is unflattering to a part of my anatomy because I just don't see right in regards to self image?