as a groomer, I see this often. Mostly it is on "fat" cats. I have seen a drastic improvement in coat condition by simply changing to a premium food, as was stated earlier. And I think the reason larger cats get it mostly is the effort it takes for them to reach around and clean themselves. Definitely look into changing her food! But also consider bathing (take it to a shop if you cannot do it). You would be amazed how well there skin reacts to a thorough washing.