From Fedora Project Wiki
- XZ compression is now being used for live images. This results in about 19% better compression for the Desktop spin and about 9% better compression for the Games spin, allowing us to put more content on these spins. If you are making your own spins, you need to be aware that XZ compression is now the default, but will not work on kernels before 2.6.38. You can use the --compression-type=gzip on livecd-creator to get gzip compression which will work on older kernels.