*WhistleeeeeI'm pretty sure this is about proportions. You can have a big bum for your proportions, no matter whether you're skinny or fat.
When I gain weight, I see it first in my bum and thighs. Last year, I was very stressed (first job, etc) and gained about 15 lbs. I gained almost 2 sizes in pants, but stayed the same size in shirts. I guess it's called "pear shape", and it's long been known to be somewhat healthier than "apple shape", i.e. where you have a lot of weight in your belly.