[surfearth]

In practice, US banks are most certainly able to grow at over 25% a year. This is most typically done via series of acquisitions rather thank organic growth. There is an undefined upper limit placed on inorganic growth by regulators, but this has more to do with ensuring the proper integration of acquisitions rather than a fear of growth in general. The reason one does not see banks grow organically much faster than 25% pa is because (i) owners can not put raise equity capital at a sufficient pace and (ii) rapid loan portfolio growth (on the asset side of the balance sheet) typically results in a deterioration of underwriting standards, this in turn requires additional capital and increased scrutiny from regulators.

Source: I've worked in bank-focused private equity for the past decade both in and out of the US.