From the time Bank of New York Co. and Mellon Financial Corp. announced plans to merge, in December 2006, until they completed the deal and Bank of New York Mellon Corp. was born, in July 2007, the banks’ combined assets under custody ballooned by $3 trillion, to $21 trillion. BNY Mellon has picked up a further $2 trillion from the merger through March 31. The growth is attributable at least in part to pension funds, mutual funds and other big investors eager to avail themselves of the economies of scale that only a custodial behemoth can offer. But smaller customers wanted in on the action ...