I just took it as a sign that someone was complimenting my ability to grow a healthy baby. Most of the people I saw who said 'gee, you're big' meant 'big compared to the last time I saw you', which is fair enough really.

But for those who aren't very polite about, try:

To 'gee, you're big': 'gee, you're rude/annoying/obnoxious, but I don't feel the need to call your attention to it, do I?', or,

'sure there's only one bub in there': 'yep, I'm sure, because I'm clever and can count to one', or 'yep, but I needed somewhere to store the pram too, it won't fit in the car'

'wow, you're going to give birth to a toddler': 'actually, I've decided to keep him in here until he's eighteen and can legally kick him out onto the street. Much easier that way', or 'no, an elephant actually, didn't you read about it in the papers, it's the first ever cross species surrogacy. Pretty cool isn't it, what science can do these days'.