Interest on excess reserves explains much of that change.
In the olden days of 2007, banks would only hold as many reserves at the Fed as they legally needed. They would instead invest any extra (or make their deposits less attractive, so that they have fewer excess reserves).
In 2008 the Fed started paying interest on reserves above the legally minimum (so called excess reserves).
Excess reserves parked at the Fed don't contribute to the economy, but are still counted in the Fed balance sheet.
You can see on https://fred.stlouisfed.org/graph/?g=15mr6 how the level of reserves at the Fed shot from about 0.050 trillion USD in 2007 to about 3 trillion USD today.
In the olden days of 2007, banks would only hold as many reserves at the Fed as they legally needed. They would instead invest any extra (or make their deposits less attractive, so that they have fewer excess reserves).
In 2008 the Fed started paying interest on reserves above the legally minimum (so called excess reserves).
Excess reserves parked at the Fed don't contribute to the economy, but are still counted in the Fed balance sheet.
You can see on https://fred.stlouisfed.org/graph/?g=15mr6 how the level of reserves at the Fed shot from about 0.050 trillion USD in 2007 to about 3 trillion USD today.