Normally the must common compression on games ould be texture compression, but havent heard anything that goes byond 6x and also texture compression introduces artifacts depending on the scale of compression, but in this ase since shinen had to work with files bigger than 1GB per mesh, with texture compression would have been impossible to fit the game in Wii u since the console only has 1GB of RAM and 35MB of Edram; the only think i can think off to reduce the memory requirements for the meshes is tesselation+dispalcements cause reduces memory requirements by hundreds of times, we can consider it like some kind of geometry compression that with a mesh of only thousands of triangles can approximate the look of a mesh of a mesh of million of triangles and since it happens in real time the storage thats being used is that of the low poly mesh and not the transformed one that now is a high poly mesh
