I don't think it's about actual weight that you are - it's about the change in weight relative to what you were. I'm naturally slim and have always hovered around the same weight rarely gaining/losing much no matter what I eat. I have a boy, but then I also had a very blue diet without knowing it and always tried to eat a good variety of nutrients.

A slim person isn't more likely to have a girl than an overweight one. If the overweight lady is dieting and losing weight, and the slim person is getting even healthier/is healthy the overweight lady is probably more likely to sway pink because her body will think "Times are hard - I'm not getting as much to eat any more" and that sways pink.

Personally I'm losing as much as I can without going underweight (that was 10lbs before my diet), so losing it slowly and hoping that my weight loss will naturally plateau when I've lost a little more.