I put some of my images in multiple galleries, not to make the galleries look bigger but because if they fit more then one theme then that's a second chance for it to get seen. If it would work in two galleries, but I only have the image in one, and customer happens to go to the gallery that doesn't have that image, it could be a missed opportunity. But I don't do it with all of them, and I usually only put it in no more then two or three galleries, even if it would fit in more.