When compositing several images into one document I often find that a portion of a layer will end up being positioned outside of the visible image area. Photoshop, of course, is still keeping track of this information (in case I choose to reposition the layer), but when I’m certain that I will no longer need it, I will choose Select > All and then Image > Crop. This eliminates unnecessary information outside or beyond the visible image area and will typically help to keep my file size more manageable