The title of the song is a kind of joke/proverb. For instance, many rock bands that are well past their prime continue to be "Big in Japan" long after they cannot draw a large audience in the UK or US.
However, this song is mostly about a couple, who tries to get off a heroin addiction. The lead signer Marian Gold explains this this way:
"I must admit the only ones who understood the meaning of the song correctly were Bernhard (Lloyd) and our then publisher Andy Budde. .