This number (the $73.7bn) is almost entirely meaningless. If you look at it from first principles, the US federal government does not need to maintain any accounts at all, since it is the entity that controls the monetary system. When they want to spend, they can just credit bank accounts directly; the money doesn't need to come from somewhere.
However, partly to make the payment systems run in a more unified manner, and partly for simple historical reasons, the Treasury does maintain accounts with a balance in them. Since the flows in and out of these accounts are important for maintaining the inter-bank interest rate, any movements there are closely coordinated with the open market operations of the Fed. In the end, the number in those accounts is basically whatever happens to satisfy the current monetary policy best.
To sum it up, the comparison in the article is void of meaning.
For understanding these kinds of things I found the writings of Modern Monetary Theory very interesting. See e.g. here: http://pragcap.com/resources/understanding-modern-monetary-s...
There is also an entire chapter in the book "Understanding modern money" by Randall Wray devoted to the accounting contortions that the Fed and Treasury do while running the payment system.