To make a Christmas tree fuller, you can try a few different methods. One option is to add more lights to the tree, which will create a more vibrant and lush appearance. You can also try adding ornaments or other decorations in different shapes and sizes to fill in any empty spaces. You can also try using different types of trees, such as a fir or a spruce, which are known for their full and fluffy appearance. Additionally, you can try using a tree topper to add an extra layer of fullness to the tree.