Industry standards want an even flatter surface for big tile than for smaller ones! The only potential saving grace is that unless they are VERY flat with no cupping or warping or twisting (put two of them together face-to-face and check), you should be using a medium bed mortar to install them, and that CAN (not necessarily needs to) be reliably thicker than thinset. Keep in mind, thinset is NOT designed for anything thicker than 1/4" thickness installed (nominally, 3/16" is the design goal with most). If applied thicker than that, it is subject to shrinkage and cracking, not something you want underneath your tile. A medium bed mortar is designed for thicknesses up to about 3/4" or so (while there's an industry spec on thinset, there is no testable spec for a medium bed mortar on max thickness...IOW, you have to go on the manufacturer's installation instructions, and that varies by brand and product). You can only reliably get a good bond and a good result with a thinner layer when the tile AND the surface are perfectly flat and plumb...that can happen, but it is not the norm.