I managed to figure out how to change the projection angle setting on my maps (it’s tricky if you’re using the App Store version of BaseCamp, so I might write that up somewhere). If I set several maps to the same projection angle, I do get the same scaling, so that’s definitely what was going on.

All I’m missing is an idea of what the “projection angle” means… I thought if, for example, I set it to my latitude, that might correct the map so that square features on the ground showed up square, but that seems to be very far from true (everything is very stretched horizontally). Do you know of anything that describes which angle this represents? Googling around just tells me that there are a lot of different map projections, but I can’t find which one Garmin uses, or what parameter this might represent.