Depending on your tension and whether your hair was wet or dry when you began to twist, your twists may be skinny or more puffy. One of the reasons the two strand twist style works better on natural hair than relaxed hair is that curly or coily ends will hold themselves in place without much help (if any). With straight hair, the ends will unravel because they don't have anything to "grip" onto.
If your hair is relaxed, you can try twist extensions to see how you look in them and if you like the look.
