I dunno. {i]Blade Kitten[/i], for example, was overall very short, but each of the individual missions were generally too long, I thought...whatever new thing you were doing in each level got tiring long before the level ended.
On the other hand, Spiderweb games such as Exile 3 featured massive maps, and with one or two deliberately sparse areas, all were full of interesting things. Really made the world come alive, though sometimes sticking too much into too close an area feels a bit odd. Generally, it helped Spiderweb games, but if the story and world-building weren't there, I don't think it would have really mattered much. Sometimes, though, it was easily to lose the point of the game, and be wandering round in the world, if you see what I mean.