There are two separate things going on here; although both of them intended to relieve pressure in the money markets.
Firstly, there's a so-called "reserve management operation" which is intended to increase reserves held at the Fed by purchasing Treasury bills. Increases in reserves implies greater liquidity to support money market activities. This definitely shows up as a growth in the Fed balance sheet, as the policy is to roll into new Treasury bills at maturity.
Secondly, there's an ongoing campaign of overnight and term repo operations where necessary to achieve the policy target rates. These are short-term, and do not result in a significant balance sheet growth.
