Why a large proportion of a bank’s balance sheet would fall into either Current Assset or Liabilities, while defining Working Captial?

In "Valuing Financial Service Firms" by Aswath Damodaran he wrote

With working capital, we run into a different problem. If we define working capital as the difference between current assets and current liabilities, a large proportion of a bank’s balance sheet would fall into one or the other of these categories. Changes in this number can be both large and volatile and may have no relationship to reinvestment for future growth.

Why do we have such distribution where one side has more (things, I don’t what to term them i.e. assets and liabilities in general) than other? Is it because how an asset or a liability is defined when used in the context of Bank or any Fin. Serv. Firm?

BTW to just clear things out. This is not homework assignment, I am a Physics student but reading this paper out of curiosity but since I lack the formal background in the subject, May be I got stuck on couple sentences. Appreciate I your help, so that I remaining part.