If your background layer is 3 MB, and you make a duplicate layer, the new layer will also be 3 MB, thereby increasing your file size to 6 MB. Each time you make a duplicate layer, it adds the size of the layer you are dulicating to the file size. Instead of making a duplicate layer, use a new blank layer for cloning, painting, or anything else that you can use a blank layer for. A blank layer has no size until you do something to it, thereby adding little (in most cases) to the file size. Or, you can go out and buy a lot more ram, and a bigger hard drive if you would rather do it that way.