oh the same thing happened to me. I told a girl that I'm pg, she looked at me and said that I must be about 2 months. I was 4.5 months then, and when I told her that, she seemed really surprised. She said she thought I would be huge at 4 months.
I'm sure it's just because people have no idea about how fast the bump grows. I was the same actually. If I thought about a pg girl, I instatntly would have a picture of a girl with a huge bump in my head. I didn't realise that it's only in the last couple of months when you become so big! The only difference is that I wouldn't say anything like that to another girl. But I'm sure those who do mean no harm!