Or maybe it's really just the game itself which is the problem. I mean, let's say the game would be multiplat on XBO+PS4+PC. Would this kind of  game which is considered by most reviewers and also players as "average" really sell millions on PS4 compared to the 500k-1m it will sell on XBO physically? 

It would sell probably around 1.5m phsically on PS4 (and it will sell like half of that on XBO) but I just don't see why this game is supposed to be a big seller on another platform and how XBO players are to judge. 

It simply isn't a game which would sell millions on any platform as long as they won't throw it on us for super cheap and/or bundled on holidays with a  physical copy  or as long as it woudn't be so hyped as exclusive that people would buy it only because of hype (which wasn't really the case with Quantum Break, not like how The Order sold probably 50% only because of that)

When I look at the PS4 list of games which sold between 1-2m I'm pretty sure that Quantum Break would land anywhere there and selling 2x as much as on XBO is not really anything crazy considering the much bigger userbase. It wouldn't sell 2m+ on PS4 and only then you could maybe say that it's the fault of the XBO players and how the game would be more successful with another userbase. 

If it would be a JRPG or something then yes, it would probably sell 5x as much on PS4 (or even more) but I think Quantum Break just isn't the right game for many people regardless of the platform. 

I get what you're saying about the userbase numbers. Not just this game or generation though. LO and Alan Wake are examples. I'm saying I wouldn't put my game exclusively on MS if I'm a outside developer. Why can't we make these types of games million sellers? Hardly anyone even tries the games. What numbers do you think Scalebound will pull?