Even the big players want to have their own physical control of their gold bullion. They are terrified of some type of govt. nationalization or freeze.
HSBC is the custodian for the GLD trust (the ETF that tracks the price of gold). Whenever demand for shares of GLD causes the price to go above the price of gold, new shares are issued and more gold is bought. So if you look at the number of outstanding shares of GLD, and thus ounces of gold that should exist in custody, you can see that there has been a lot of demand out there. So my guess is that a big chunk of the need for space is probably from HSBC’s own activity as custodian.