Bank of America’s Super Size Me diet may weigh it down.

Bank of America is also more than happy to introduce another cog to its machinery. B of A’s philosophy is that Merrill adds investment expertise that allows B of A to diversify its services. The more services it has to offer the average consumer, the more integrated the company can become. When it acquired Countrywide, the bank said customers who buy home loans from B of A buy three times the assets as nonmortgage customers. Expect that number to jump higher when investment services are lumped in. Some suggest that in a credit-crunched economy, the more diversified your assets, the safer you’ll be if economic disaster strikes. But there is such a thing as having too many moving parts. Just ask Citigroup.

After lobbying to get the Glass-Steagall Act repealed in the ’90s, Citigroup solidified its position as a conglomerate that swam in both the consumer- and investment-banking pools. A decade later, though, Citigroup’s stock was worse off than it had been before the mergers, and the New York Times wrote that “bloated costs, outmoded technology and political infighting have hobbled the giant company.” Cue the subprime-mortgage fiasco, which forced a company as diverse as Citigroup to lay off thousands and post loss after loss. It continues to grapple with write-downs and unsellable assets.

In many ways, Bank of America’s purchase of Merrill Lynch is a sequel to an earlier fruitless adventure with investment banking. It took a crack at an in-house operation in the past but folded the department last year after it sunk $625 million into the effort and ended up in the red. In 2006, B of A’s CEO, Kenneth Lewis, told the New York Times that he didn’t want B of A’s investment arm to be like the rest of Wall Street. Lewis continued, “They have too much proprietary trading to be sustainable and have consistent earnings.” Yet here Lewis is, absorbing an unsustainable investment house.

So then why enter the fray in the first place? Surely, B of A has convinced itself that Merrill has real cost-savings and synergy potential. But the real reason the deal happened is because Lewis so badly wanted an investment bank to call his own. Fresh off the closure of the in-house i-bank operation, Lewis was asked whether he was interested in Bear Stearns. Lewis snapped, “I’ve had all the fun I can stand in investment banking.” Except it wasn’t he who had had enough of the fun; it was B of A’s ledgers. This summer, as the financial world around it crumbled, B of A was one of the few corporations to retain stability. With a new landscape at play, it was fiscally prudent to start having fun again.

That some are grumbling that Lewis overpaid for Merrill only underscores his lust for investment banks. Lewis can’t help himself. He has a compulsion to acquire, no matter the potential health risks. We’ll see if shareholders have the stomach for it.

(Photo by Davis Turner/Getty Images)