I'm not sure there's any rhyme or reason to it TBH. I only put on 6kg and DD was almost 4kg (8lbs 10 oz).

Genetically, she was not meant to be that big either as DP and I were both skinny as kids (not as adults because we both eat and drink too much). So DD was born on the 90th percentile and has since slid down to about the 10th so she's got back to where she was meant to be in the first place.