Lore wise, i'd like to think that there are more than 15 workers on the island.... Maybe a couple couple hundred thousand workers/people living on the island... A jmod said somewhere that there are at least 2000 varrockian guards guarding the borders of the city, lore-wise, but in game, it may look like their are perhaps 3 guards at each entrance to the city.
Maybe there's more than one adventurer, too? I mean, one dude liberates all the cities, acts for all the gods, and kills all the monsters? No way. We're clearly a massive hero team - sharing that big pile 'o geepeez.