We've been around 1mloc for the Drakensang single player and MMO games, and that was 15..5 years ago, with a relatively small team (up to 20 programmers), and budget-wise far away from what's considered an AAA production.

While I wouldn't want to work on a 10mloc C++ code base either, it sounds totally realistic to me.

realistic, but probably far from "optimal" (not that LoC is a good target for anything other than complexity). There simply isn't much incentive to properly refactor legacy code into something more managable. Rather eat the cost with devs ramping up and working around the cruft.