I don't tell anyone my weight either... and it drives my hubby CRAZY!!!!
We're both trying to lose weight adn get healthier--- I'm the one who inputs our numbers into our weight loss spreadsheets--- so I know his but he doesn't know mine (but he's a GUY!)
I think for me it started b/c I'm so tall that even as a fit kid, my weight was SO much more than the other shorter kids that they didn't CARE why the number was bigger, just that it was bigger.
Even as an adult... I'm 6'1"--- I'll NEVER be 100 pounds. My goal is 200. To most short people they would GASP at the thought of 200 POUNDS being a "goal" weight.
But that would put me at around a size 12. Pretty nice, if you ask me.
So I don't tell people b/c a lot of them don't realize that a big NUMBER also correlates differently when you have a big HEIGHT. Me, too, the doctors always starts the stupid scale WAY lower than it should be b/c I'm tall and "carry it well". I always just reach forward and move that big sucker up a notch.
But yeah--- unless people are just as tall, they have this idea that "thin" equals less than 150 pounds or something. So I just don't tell anyone.
I do report my LOSSES--- and I'll tell my clothing size (b/c I've never been bigger than a 22) but it's the LBS that freaks people out. So I just don't say anything.