You're right, completely normal. I've had leather from every american (auto) manufacturer and they all did this. You'll also notice "seat crush" in the same area for the same reasons. Usually the taller the vehicle (or the shorter the person) the more you'll see it unless you step on running boards and sit directly on the seat without sliding across. It isn't really a weight thing though that speeds it up I'm sure. I'm 6'2 @210 but was 300 so have seen a wide range of conditions.