Big has never been badder. In the US, President Barack Obama wants to add to the existing 10 per cent cap on a bank's share of deposits. Another cap, targeting total liabilities, is on the cards. If 10 per cent remains the magic number for whatever reason, it does not really matter what is targeted – assets, liabilities or even size relative to economic output – the top three US banks would probably exceed the cap. Together they even have a bigger share of total liabilities, say, at about 45 per cent, than they do of deposits, at 30 per cent.

In spite of the tub-thumping, the US is not planning to force the largest banks to shrink. Nor will it prevent big banks growing organically. This seems perverse. If banks' sheer bulk is a systemic problem, then a new cap that preserves the status quo is merely window dressing. But even insisting that banks become smaller on a relative basis as the sector grows would stifle loan growth from the big boys while allowing the whippersnappers to go for broke. What is more, market concentration is a poor guide to potential risks. Canada and Australia's highly concentrated bank systems, say, sailed through the crisis.

So there is an irrational fear of big. But there is an aversion to some small businesses too. Former Federal Reserve chairman Paul Volcker's disdain for proprietary trading stems from the principle of what a government safety net should protect. In reality, prop desks played little part in the crisis. Goldman Sachs estimates that plain old bad lending accounted for 95 per cent of bank losses. A 2008 World Bank study found that greater market power can actually reduce risk-taking as a bank's franchise becomes more secure. The study did suggest that risk within loan books increased with banks' relative size. But that can be controlled by upping equity capital requirements, limiting leverage and forcing assets back on balance sheet. Reforms are already taking this path. Regulators should remain focused on risk and capital – bigness is a sideshow.