Large-format tile looks clean and modern until it's set on a wall or floor that isn't flat. Then it shows every dip the small tile used to hide.
Tile size has grown steadily more popular over the years, and the substrate requirements underneath it have grown right along with it, whether or not that gets explained at the time of the quote.
A four-inch tile can span a minor dip in a wall or floor without much visible consequence — the grout lines break up the surface often enough that small variations get absorbed. A twenty-four or thirty-six inch tile spans the same dip in one uninterrupted plane, and the unevenness shows as a visible edge, shadow line, or rocking corner.
Lippage — one tile's edge sitting proud of its neighbor — gets blamed on the tile more often than it should. In most cases the tile is fine and the substrate underneath was not flat enough for that size of tile, or the setting technique did not account for it.
Getting a substrate flat enough for large-format tile usually means a self-leveling compound on floors or a skim coat on walls, checked against a straightedge before any tile goes down, not eyeballed. It adds a step and a cost that a smaller tile installation could sometimes skip.
Even on a flat substrate, back-buttering and proper trowel notch size for the tile size matter more with large format, since there is more surface area for a low spot in the mortar bed to create a rocking tile.
A bid for large-format tile that does not account for substrate prep is either assuming the substrate is already flat — worth verifying, not assuming — or planning to skip a step that shows up later as lippage across the whole floor or wall.
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