The ladder is not arbitrary. 10 mm carries telephone and data wire and nothing else. 16 mm is the smallest size most electricians will accept for a lighting circuit. 20 mm is what an installer reaches for by default and is the size a wholesaler sells most of. 25 mm is the step where a run stops serving one circuit and starts carrying two or three. 32 and 40 mm carry the distribution leg.

Over-sizing feels safe and is not free. Larger pipe costs more per metre, takes more space in the slab, needs larger fittings and clips, and takes more volume in the container, which on an export order is a real cost rather than a rounding error.

Under-sizing costs more still, but later. A conduit filled beyond a sensible fill ratio makes the cable pull hard, which damages insulation during the pull and makes any future re-pull effectively impossible. The run then has to be opened rather than re-wired.

The practical rule for a buyer is not to specify one size and hope. It is to stock the ladder, because an installer who cannot get the next size up will buy the whole job somewhere else.