So there is something to be said for regulating and supervising banks according to the proportionality principle. Here, harmonisation comes up against a boundary which is both reasonable and necessary. In public discussions proportionality is often tied to the size of the institutions. This makes the discussion easier, but it can also lead in the wrong direction.
The decisive factor should be the bank’s systemic importance. And at the latest since the savings and loans crisis in the United States in the 1980s, we know that even small banks may still be systemically relevant – “too many to fail” is as big a problem as “too big to fail”.