I used to get that all the time back in college. One girl was convinced I was at least six feet tall...
"There's no way you're 5'10", *I'M* five ten and you're taller than me!"
"Uhhh.... Take off your shoes and stand back to back with me"
"Oh"
Every girlfriend I've had says no one walks like me, shoulders back, chin up. It's hard to take it as a compliment though, considering it's not something I do on purpose, it's just the way I walk. Now, when it comes to sitting down, I currently have my butt hanging off the chair with my feet propped up on my desk, which is the same height as my head when I slouch like this.
I think it really comes from two things--most people don't make eye contact anymore, and everyone's in such a hurry to get where they're going it looks like they're halfway ready to leap into a sprint at a moment's notice.