I think I was a bit bigger than you are at 15 weeks. Got a lot of the 'are you sure there isn't more than one?' questions. My grandad said I was a similar size at 16 weeks to his step-daughter who was 32 weeks at the time! I did measure pretty large at 28 weeks, but at 34 weeks I'm measuring in the normal range (though at the top), so my growth seems to be more in line now.