Why do the names of some Japanese prefectures contain 『しま』when, clearly, they
are not island prefectures?

For example, 広島 means "spacious/vast island," doesn't it? However, Hiroshima
is not an island... And there are more examples like this: 福 **島** 、徳 **島**
、and 鹿児 **島**.

I hope you can clear this up. Thank you!
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2019-10-27T14:59:36.480
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2019-10-27T14:59:36.480
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question
etymology
Why do the names of some Japanese prefectures contain 『しま』when, clearly, they are not island prefectures?