What is the reason some video games have way more content then others?

Take GTAV or Witcher 3 for example and compare them to Starwars Battlefront or the new Rainbow 6. The latter 2 didn't even include campaigns.. I understand if they choose to opt out on it because not enough ppl played them but why can't the studios make up for it with improved graphics or a shit load of maps? (Especially when the games are damn near the same size). I'm no tech geek by any means, but cutting corners somewhere should only enhance the rest right? Or is about the studio saving money?