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

Almost lost in the smoke-bomb of Lehman Brothers’ epic fail Sunday was Bank of America’s historic takeover of Merrill Lynch for $50 billion. Over the last seven years, B of A has made $100 billion in acquisitions that have expanded its service in core banking (Fleet and LaSalle banks), credit cards (MBNA), and mortgage lending (Countrywide). Now, it’s investment services’ turn.

Soon, Bank of America will touch nearly every aspect of your financial life. It will be able to provide a bank account to save your money in, a mortgage to spend it on, a credit card to splurge with it, and now advice about how to invest it. It’s a real life, fiscally concentrated version of Wall-E’s Buy ’n’ Large.

That behemoth is only on the consumer side of the equation. Once Merrill Lynch is fully integrated, Bank of America will supersize its investment footprint as well. It will be the biggest bank in the country, by far, combining consumer and investment banking in a way that few other financial institutions manage. It adds Merrill’s $966 billion in assets to its own $1.7 trillion. For perspective, when Citicorp merged with Travelers in 1998, their combined assets were worth only $698 billion. Bank of America has eaten something nearly half its size, growing to be an even more imposing and influential force.

But indigestion seems likely. Stuffing investment services into Bank of America’s portfolio opens up a host of new vulnerabilities for the company. The more weak spots B of A invites into its business model, the more likely it is to get hit in a still-volatile economy.

Initial reaction to the deal was harsh for B of A. Its stock led the market into a deep valley, down more than 20 percent on a day that saw major indexes fall 4 percent to 5 percent. Merrill Lynch’s stock was more or less even.

B of A and Merrill held a press conference Monday morning to hype the hastily arranged deal. B of A preened about the potential cost savings, which may very well be true. (It will be hard to tell for at least a few quarters.) But Merrill also brings with it $50 billion in high-risk positions, according to a Citibank analyst. That $50 billion is largely the reason Merrill needed to be sold in the first place. Merrill will have just as much trouble getting rid of its compromised assets under Bank of America’s roof as it did when it was on its own. Analysts expect Merrill to have to write down as much as $3 billion more of its assets this quarter, even though Merrill has been religious about proactively writing them down. That burden, when compounded with Countrywide’s questionable finances, begins to get weighty for B of A.