The are reinvesting less of what rolls off (matures or pays down).

The portfolio unwind will take a decade plus. They won't contemplate a sell until the portfolio is $200B or so in my opinion. You would see a massive spike in rates/spreads if they tried to actually sell securities before the portfolio is much smaller.

I doubt it will ever get that small either. The fed has said they will continue to alter the size and composition of the balance sheet as they see fit, meaning increasing it if necessary.