Assuming you're flying at 800kmh in something like an airbus A380, you're flying 13.3km each minute, uncovering a large part of a new circle/sphere of terrain with a radius of 13km (half of it overlaps with old already-downloaded terrain). That's half of 555km squared of terrain. That's a lot of terrain. If you want that terrain to be fairly accurate, you'll want to see at least meter accuracy near the plane (if you're near the ground you'll want to see one datapoint of terrain per meter or more), with lower levels of detail as you get further away. Add onto that things like the placement of trees, bushes, rocks, and all the texture data of the terrain (probably an index into existing possibly procedural textures), and you've got a lot of data that needs to be transferred.

10Mbps seems pretty fair for all of that.

Also terrain data is updated regularly, and you might not want to keep around old terrain in the first place. There are reasons like players only flying some routes once and never again, and if you save all of mozambique for someone who actually only flies around in the US that's bad too.

EDIT: Buildings of course cost extra. Airports take up a bit of bandwidth each time you take off or land, as they are probably custom modeled. Cities like NY or LA though will have a ton of custom modeled buildings and textures, and those cost a lot of bandwidth.