I've wondered this too, but I think a lot of the time it's like when a baby is born....what's the first thing people always ask: "How much did it weigh?"
And that doesn't stop! If you run into a friend with her baby, at SOME point you usually ask, or she mentions, how much it weighs.
I dunno....it's just one of those things people like to know!
And I think that it helps people envision your pet too....it's hard to tell in pictures....a 12 lb yorkie can look just like a 4 lb. one.....so, not that it's better or worse to be larger or smaller....but it just helps you picture the dog better!
And then, there's always people PROUD of how small their dog is!
I think I mention Mazie's weight a lot, but not because I'm proud of it....I WISH she would gain weight!
PS, geez, while I was typing all that out...everyone else said what I was going to say!