I'm sure there are any number of approaches, these are some of them:
- Compression: reduce the space need on the disc at the cost of retrieval+decompression later which costs cpu usage and time
- Download as needed: get the data you need as you need it and cache/discard data that is no longer needed. Requires some kind of server infrastructure, but it may already exist and be available for your application.
- Pre-process: Take the original data and strip out anything you don't actually need for the game itself, which might pose some problems if the game isn't totally complete/planned out and so you end with a full disc and needing some additional data/metadata.
There is also always the option to go with lower quality assets (art, music) and less sophisticated code/AI. The core game/mechanic may not require that level of detail or accuracy to approximate realism and still be fun/playable. It sounds like you need to flesh out the idea somewhat and figure out exactly what is contained within the Open Street Map data. The reason is that you need to know exactly what data the game will require and if there is anything you can just straight out omit. Such an inquiry might also prove to show that the scope of the game is just too broad and should be simplified regardless.
It's almost certainly a balancing act in the end, you may actually have to immerse yourself in such a project to know beyond a doubt whether it's viable. You can also just shove the requirement at at the player and hope for the best. Most people don't balk at a 10GB game these days because their hard disks are very large. I remember that Neverwinter Nights 1 was ~4GB of disk space back when my hard drive was only 80GB. That's 1/20 or 5% of the total disk. Obviously that didn't keep people from playing it. On the other hand Team Fortress 2 takes up 18.4GB of my hard drive except that my current hard disk is a 1TB drive so it's only 20/1000 or 2% of the disk space.
I am giving PC numbers, but the PC/console difference seems less and less every year. In any case the point is that there's some hardware use target and they develop toward it intentionally, probably even more so on a console. A game requiring 7 DVDs seems rather insane to me, but if the players will buy it and play it. The fact of a captive audience that needs a game to play on their console seems relevant here.