Why the substrate matters more
A standard 300x600 tile will forgive a couple of millimetres of wall deviation across its length. A 1200mm tile will not - the deviation shows as lippage at every joint. That means walls often need boarding, dubbing out or a full skim before tiling starts.
On floors, deflection is the equivalent problem. Timber floors usually need overboarding with a tile backer board to stop movement cracking the bed.
Labour and adhesive differences
Large tiles need a solid bed with no voids, which in practice means back-buttering as well as notching the wall. Levelling clips are effectively mandatory. Two fitters are needed to place anything above about 600x1200 safely.
Budget for roughly 30-50% more labour hours per square metre than a standard-format wall tile, offset slightly by fewer grout lines to finish.
Where large format genuinely wins
Small bathrooms benefit most, because fewer grout lines make the room read as larger and there is less to clean. Shower enclosures benefit because there are fewer joints for water to work into.
It wins least on heavily broken-up walls - lots of cuts around windows, pipe boxing and niches means high waste and a lot of the visual benefit is lost anyway.
Cost impact in plain numbers
On a typical 2.5x2m bathroom, moving from standard to large-format porcelain typically adds a few hundred pounds of tile plus additional labour and preparation. If a quote for large format is identical to the standard-format quote, the preparation has almost certainly not been priced.