I haven't got a clue why they do it, I'm not a baby person, but maybe if the baby was really big, painful to carry round and give birth to, I think the poor mother has the right to say how big and heavy it was, pat her on the back!
But the fashonable skinny world we live in I can see really vain people these days saying how light their baby is, like the joke about a women being happy she is slim and saying she's back to her birth weight.
I think most parents mention it though because its like tradition, back in the days people didnt have much excitement in their lives or anything but the weather to talk about, they created all these stupid traditions we get stuck with today.