A lot of it has to do with hip size as well as weight. My friend and I for example have exactly the same size thighs but she has a much bigger gap between hers than I do, it's very noticeable. This is because she has very wide set hips, which she hates. I personally quite like them as they give her a nice shape. If someone with her size gap had my size hips however, it would be quite worrying as they'd need to be drastically underweight.