The money, and these big salaries, aren’t really in deposits, but in structuring deals, whether they are loans, public securities offerings, mergers and acquisitions, underwriting, securitizations, or any of a variety of other transactions. The people we are talking about are not the bank officers in local branches, but the people in New York, London, Tokyo and other financial centers who bring deals to market.
Oversimplifying greatly and pulling a number out of my behind, let’s say that a bank gets fees of 1% of the transaction amount for the average deal they do. So, if the bank does $1 billion dollar deal (arranges the sale of a $1 billion company, underwrites the sale of $1 billion of stock, etc.), it gets fees of $10 million. Of that $10 million, a relatively small amount of it is used to pay the overhead of the bank (offices, electricity, computers, accounting and HR, etc.), and the rest is available to pay your people and to be profit to the bank. From that deal income, a large chunk will (and should) be paid to the team that arranged the deal, who convincing the client to come to the bank, had the experience and connections to complete the deal, and actually got the thing done.
In many ways, this is just like a homeowner paying their bank a point (1%) to get their mortgage or a real estate broker a commission to sell their house. The individual professionals get paid out of those transaction costs.
A big bank will complete mutiple billions (or more) of deals in a year, and take a small slice out of that transaction flow. Of that small slice, a big portion of the slice is paid to the individual bankers who generate the deals and make the deals happen. Because the value in a bank is not really in fixed assets or equipment, it is mostly in the people, who demand pay relative to their contribution.