Big in Japan?

Big in Japan - is that an idiom and if so what does it mean?

It depends on the context but very often this expression is used to apply to film stars/ music groups and people in the world of entertainment. It means very popular/ successful in a country.

Hello Alan! Thank you for answering my question so fast. Is it true that the expression Big in Japan can also have a negative meaning? A friend of mine told me that you say that you are big in Japan, it actually means nobody really knows you in your own country.

I suppose anything anyone says can also be used in a sarcastic way but that wasn’t my initial reaction.