I think the fact that Japan is an island and that their civilization spent a good deal of its history isolated from other countries might have something to do with it. But really, languages tend to have lots of synonyms for things. There may not be any particular reason for it, any more than there’s a reason we have so many synonyms for “big” in English. Big, giant, huge, enormous, gigantic, titanic… probably others I’m forgetting.
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