I don't really know. Their albums usually sold over 600K on the first week, with the exception of last year, which was over 800K on the first week (and over a million copies overall). I doubt even Sandaime can even compete with Arashi.
But think in this way, she's only beaten by two boy band/group (one is really popular right now, the other one has a huge fanbase). It's not that bad.
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