Japan's Sumo Bank

From electronic gadgets to bonsai trees to dinner portions, the Japanese are famous for doing things small. But in sumo wrestling and banking, gigantism is the order of the day. At a press conference last week, officials at UFJ Holdings and Mitsubishi Tokyo Financial Group (MTFG) announced they had agreed to begin negotiating a merger that could create the world's largest bank, a monster with $1.75 trillion in assets, overshadowing Japan's Mizuho Holdings and U.S.-based Citigroup, both with approximately $1.3 trillion in assets as of March.

The deal, if approved by regulators and shareholders, could create a...