Big clients have monopoly power

When big banks underwrite corporate debt, they take on some risk and cost.  Big clients buy large amounts, it reduces the sales queue, hence costs are more predictable.

Bloomberg: When issuing bonds, companies typically hire syndicate bankers at large dealer-banks who are charged with gauging demand for such debt, estimating a good price at which to sell the resulting securities, and identifying clients who might be their best buyers and holders.
Such bankers must weigh a variety of factors in their decision-making and sometimes may underprice new bonds in order to make sure they end up in the portfolios of large ‘buy and hold’ investors who are seen as more dependable. So-called ‘concessions’ on new issues may also arise as investors and banks involved need to be compensated for the extra risk of holding corporate credit as opposed to safer securities such as U.S. government debt.
Fideres found that the price of corporate bonds typically increases by 0.50 percent between the day the price of the new debt is announced and the day it’s sold, compared to a 0.15 percent rally in newly-issued U.S. government bonds in the same time frame. Bonds sold by junk-rated companies with more fragile balance sheets tend to increase by 1.5 percent, while top-tier investment-grade corporate debt enjoys just a 0.40 percent rally.