Daniel and I continued this via PM. When I opened his file, everything looked as expected, but he posted screenshots showing that Print/Graphics was not displaying correctly for him; it looked like the page size defined in Layout Options was being scaled to fit on some other page size, instead of displaying exactly as it does in Engrave.
I noticed from his screenshots that he didn’t actually have any printers installed (because he doesn’t own a printer). On a hunch, I suggested that he install some virtual PDF printer, just so that something shows up in the printer dropdown. When he did that, and restarted Dorico, Graphics preview displayed correctly for him.
It seems to me that this might be a bug – that Graphics preview requires at least one printer to be installed in order to calculate its display correctly.
cc: @dspreadbury