Not necessarily. Remember the Prime games and Batman Arkham Asylum had explorable worlds that were large, but not overly so like in Arkham City and Origins.I can't think of many open world games that have the kind of artstyle we're all dreaming for. It's tough to that stuff in open world, a lot of times you get a ton of procedurall generated hills and foliage.
It's all about scale, and how good you are at planning your budget.
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