They want to keep talking to you, because it's rude to stare at someone else's baby for that long without striking up a conversation with the mother! And your bub is probably cute and people want permission to pay attention to him.

I'm with Sherie, I don't mind saying Riv was 9.4lb, but the comments afterwards really annoyed me.
'Oh, but you were tiny, how did he get that big?' - Magic. And he worked out in utero.
'Bet you won't be in a hurry to do that again!' - Well, I'll wait at least 9 months I think.
'Ohh but he's worth it all the more isn't he?' - Oh yes, he's far more valuable than a smaller baby.
I'm thinking about starting a book with the next one, where I'll write down all the things I wish I'd said but was too polite to.

The people who make these statements usually really are lovely though, and are just trying to find a way to express their care and interest. I just kind of try to feel encouraged by the tone of their voice and not pay too much attention to the specifics.

I do understand your pain though, because like Nadine pointed out, when you're in a certain situation, you get asked the same question ALL the time, and you just want to talk about something else! I guess just try and tell yourself, they mean well, and it's not their fault that 20 other people have already asked you the same things.