Any cross- section scrap is -projection none ans as such it should be calibrated. If you calibrate it in scale 2 it will be 2 times larger.
OK, at least that means I didn't miss something obvious :) "Obviously the scrap scale can be set to twice what it really is, but this means that the data itself needs to be falsified, instead of just the layout." This also means it is not possible to have the same cross section scrap used in a partial map of part of the cave, with scale set as 1:1, and also used in the overall map of the entire cave with scale set as 2:1. It would have to be drawn twice to allow that, once with each scale factor set in the calibration. This is a bit limiting for our use-case. But OK, I can work with it, it's just a bit inconvenient.