When you gain fat, your body allocates it as far away from the vital organs as possible- bum, hips, legs, upper arms, face, as none of that really matters to the operation of the body. However when you get really overweight, the body runs out of places to store the fat and it ends up going INSIDE you, around your internal organs. This is why you can't feel or see that type of fat. When you lose weight, your body naturally wants to get rid of it from the internal organs first- so it goes from there, meaning you seem like you're not losing weight from the bits you care about (bum/legs etc) but actually it's sorting your innards out first!