To make a Christmas tree look fuller, you can add additional decorations to fill in any empty spaces. You can use ornaments, lights, tinsel, garland, or even small toys to add volume to your tree. You can also try using branches or twigs from outside to add more texture and depth to the tree. Lastly, you can also try using tree toppers or other decorations to add a focal point to the tree, which can help to create the illusion of a fuller tree.