It sounds like you might be inferring that $100 in deposits is both a) $100 owed to their banking customers and b) $100 in cash. It is only the former, though

It sounds like you might be inferring that $100 in deposits is both a) $100 owed to their banking customers and b) $100 in cash. It is only the former, though - the cash was lent out to make the loans or used to purchase income-producing assets. A bank with $100 in deposits and $50 in loans is insolvent - it owes $100 to its customers whenever they decide to withdraw it, but only has $50 of assets (loans) to back that. In the reverse, the bank only owes $50 to its customers, and meanwhile has $100 of assets - if the customers all wanted their money back, it could sell the loans, and as long as it got more than half of its nominal value they'd be able to repay everyone.