Ana, Maya didn't roll over till she was 7 mths, and by that stage she was grossly underweight and was in the 3rd percentile for weight. So size has nothing to do with it. In Maya's case it was just sheer laziness
Perhaps I am a bit of a cow, but whenever people made comments about Maya's size (as a newborn I got "oh she's so big" and at 7-10 mths I got "gosh she's skinny") I bit straight back. One old bag stopped me walking along the road and asked how old she was and I said 2 weeks and she said "oh, she's very big for 2 weeks" so I replied "I know, isn't she beautiful, and I didn't even have to have stitches after the birth". She went bright red and didn't know what to say, and I was in a great mood for the rest of the day!
We didn't do coffee group, but must admit that I felt a little inferior to the other Mum's in my antenatal group as they were all married with houses and planned babies, and I was 21 and having a big surprise. So when we got together for the babies first birthday it was nice to see that Maya was the only one walking (and she was the youngest by 2 mths!)