Wow, for them to try a guessing game is a little rude. I don't think it's right for them to do that about an issue like weight. For people it can be a sensitive subject.
For me personally, I don't mind if people ask me how much I have lost once they are close friends/family. At the moment, I just say a few pounds (people see it, I don't because it's only a small number) because it is. But I think once I lose like 10lbs, I will tell them that is what I have lost. I look at it like this: You have worked your butt of too get to where you are, if people ask, you should be proud to give them the figure you lost!
I do agree with acquaintances, it might not be as easy. For me, I'm thinking of acquaintances as people in my college class because my previous job there were only a couple of employees and I wouldn't have minded telling them. But for acquaintances, I would just say "Sure, just decided to what what I was eating and dropped a couple of pounds."
At the end of the day, this is your business, your journey and you should have the right to tell who you want and what information you give them.