The bigger the tile, the harder it is to get a perfectly flat installation. the objective is to keep all tiles in plane so that there are no lips from one to another. On some tile, this is impossible, since the tiles themselves aren't consistent (have warps, cups, twists to them so even on a flat surface, they won't lay flat). The only way to handle those is with a wide grout joint and let the grout bridge the difference.
Assuming the tile themselves are flat, then there should be no lips between them. Any bump, ridge, etc. the tile sits over can make this much more challenging, and a prefectly level floor makes the job much easier to get a good result. This can be done when setting in the thinset by building it up where needed and thinning it out as well, but takes much more time and skill (and materials in the extra mortar).