It means you have more quality than quantity; because quality is spiritual, and quantity is physical. But it also means, by way of the 15 calories, that there is a cost to virtue (or high moral standards).The Calories God affords you can either add fat or add strength depending how they are used. Thus, the 15 calories symbolizes your obedience to do as He makes clear for you to do; It means just the right amount of energy supplied by God to overcome.
Purity is something that has been tested and tried and has triumphed, something that has character at the back of it, that can overcome, and has overcome. Virtue is acquired and so is purity ... with the purity Jesus Christ puts in you He can take you where He went Himself, and make you capable of facing the vilest moral corruption unspotted; He will keep you as pure as He is Himself. You need His strength (15 Calories lol) therefore, as you start out pure by His sacrifice on the Cross, and overcome by moral strength He empowers you to overcome in His strength and character - an inclination to go the other way!
You are therefore not simply the act of morality (15 calories), but the act based on the (purity) in the the Gift of God His Son - Jesus Christ Himself as your own new life crossed over from death unto life..for now you are reborn! That is not just low-fat My Queen Aussie-ette, it is no-fat, and totally efficient. Smile. Thus, you are now my "Pure Sugar" as a nickname: The will of God found in you!