This town ain’t big enough for anyone.
A play on the phrase "this town ain't big enough for the two of us" --- this is a thing cowboys say to each-other to insight a duel, and refers to the idea that the two combatants each have big egos and only one can survive. But here, the "town" is a tiny island, and even the lonely cowboy can not physically fit. So, the phrase is: the town isn't big enough "for anyone".