Flying Arrow hit it on the head, as did Jessweeee but I'll paraphrase her statement and address it differently. The world has to feel like a world, which is where game structure comes in and why I have issues with games like X, XIII, Xenosaga Eps 1and 2 who all have great detail in the background of the worlds but the game never really throws off the trappings of you're only direction is the next story point, and the game does little to create even the illusion of choice.

Mostly, its not the size of the world but the detail that fills it in world design (story) and the amount of things you can do within it (gameplay). I generally feel shortchanged by a RPG if it is just a plot and battle system and there is little else to do. Course their are some nit-picky issues as well (the relevancy of said quests and mini-games) but that is another topic entirely. I also feel it's nice to occasionally have a town that serves no story/quest purpose. XII is the first game to actually feature towns that had no bearing on the plot since FFIX, and FFV was the last game in the series that had a town that had no story purpose or quest purpose at all, it just simply existed to give you a breather while you walked the globe.