One of the central tenets of Marxism is collective ownership of the means of production. An old-fashioned capitalist society has two social classes: capitalists and laborers. The capitalists own the means to produce goods – the factories, the farms, the mines, the machines. The laborers own their labor, and sell it to the capitalists, in exchange for wages.
Under Marxist theory, capitalists are exploiting laborers by using their capital (companies, factories, etc.) to make money without performing labor. Hence, Marx advocated eliminating the capitalist class, and having the means of production owned collectively. Instead of one Rockefeller or Carnegie getting all the profits from their company, a million people could own the company together and share the profits.
But what does that have to do with investment banking? The main function of investment bankers is facilitating the sale of company A to company B. If Widget Co. buys Gadget Co. for $5 billion, the investment banker might negotiate and do the paperwork in exchange for 1% of the purchase price, or $50 million. $50 million is a lot of money, so investment bankers get rich. And these rich, powerful people are highly incentivized to make sure as many companies get bought as possible. The more acquisitions and mergers, the more $50 million paydays they get.
The obvious result is companies buying each other out over and over and over, growing into huge conglomerates. (Here is one famous example.) Studies show that these mergers usually don’t make money for stockholders, but they happen anyway, largely because of all those investment bankers.
A company worth $1 billion might be owned by just one person, or by a few people working together. But a company worth $100 billion can’t be – it’s just too big. Such companies are almost always owned by thousands of different stockholders, often through intermediaries like index funds and mutual funds.
Hence, by making companies huge, investment bankers (largely unintentionally) are moving society towards Marxism, by encouraging collective ownership of corporations. There’s no Bill Gates or Henry Ford who owns Pepsi; instead, we all own tiny little pieces through brokers and pension funds. This doesn’t eliminate inequality, since the proxies for owners (institutional investors, board members, and CEOs) often have more power than the owners themselves. But that’s another story.
The Rationalist Conspiracy 
Marxism and huge corporations have other connections
– In trading with the West, the Soviet Union preferred to work through big capitalists: Similar mindset.
– A huge corporation does not work internally by free-market principles; internally, e.g., when one division supplies materials to another, there is a command economy. Failures occur similar to those in the Soviet economy.
Great point about investment banks facilitating conglomeration; that hadn’t occurred to me. However, I’m not convinced that increased conglomeration means more collective ownership of corporations. It seems to me that if you add up all the market capitalization of publicly traded corporations, some fraction is bound to be controlled by the public, and it shouldn’t matter much whether that market capitalization gets chopped up in to a few large conglomerates or lots of smaller corporations.
(Factors that I imagine might actually increase the fraction of the total market capitalization that’s controlled by “the public”, whatever that means: more even wealth distribution, decreased trivial inconveniences associated with getting in to the market, popular advice regarding where to put your retirement money.)