Continuity mistake: There are several shots of the island used in the series. In the opening, it appears small. In some inland shots it appears immense and mountainous, despite being described numerous times as a "tiny island somewhere in the Pacific."
Continuity mistake: In the opening credits, the boat used as the Minnow changes. It goes from having an upper steering deck, to flat topped and different colored, and back.
Answer: Bob Denver answered this when he was on a talk show hosted by Pat Sajak. He said they were going to write into an episode where the castaways found all these clothes, but it got too complicated, so he said, they decided, "who cares, it's only a TV show," and forgot about it. So the writers and actors even asked this, they just never bothered to address it.