I've been everything from a size 4 with gorgeous skin, a size 4 with horribly dry skin, to a size 20 with extremely rough dry cracked skin and also a size 20 with supple soft skin.
I think there are just way too many factors to have any real coorelation. How much water a person drinks, what their diet is, do they smoke, do they drink alcohol, do they have any nutritional deficencies, how dry/moist is the air in their environment, what sort of soap, laundry detergent do they use, do they have any underlying medical conditions etc.