People who focus on monetary statistics often forget the underlying data may be more robust than is captured by these aggregates. In the current situation there are people who point to the fact that the monetary base hasn't grown much or has even fallen for a few month over month periods in recent times thereby justifying low inflation expectations and the need for the fed to combat an apparent deflationary downside risk.
The problem with this analysis is over simplicity. The factors missed out by the statistics include a qualitative view of the quantities, a theoretical interpretation of the data over broad times, and an assessment of the recent market demand for money and loanable funds. I'm gonna try split these into three posts to keep them short and to the point. For this one, I will focus on the qualitative view of the quantities.
The Federal Reserves' ability to effect the money supply is the key to their power and influence. All policy is essentially enacted by means of this tool. The current monetary structure is arguably the Feds' most powerful environment over its history and this is partially the result of several big pushes towards providing more elasticity and therefore control over the money supply. Under a gold standard compared to today, the fed could contract the money supply more powerfully than expand, while today they have a stronger ability to expand than contract. Thus far, under the post-Nixon Fed, they have had a reasonably strong power to do both as they please but the means to effect the downside may become increasingly constrained due to macro changes and risks and changes in the quality of the Fed's balance sheet.
To see this, one must simply realize, that once the things the Fed buys to effect an increase in the money supply are not scarce, there is little to no limit to how high this can go. On the other hand, selling those same things will depend on the demand for these items in terms of how much the money supply can be effected on the downside. Default risks also threaten the ability of just letting an agreement or loan expire. Note that there is little external to the Fed that is necessary to control the upside of the money supply, however, factors beyond the Fed can disturb their ability to bring the supply down.
Consider the situation now a days where the Fed generally mostly holds high quality debt paper. The reason the fed has in the past been discriminating over the quality of paper they hold is because they want to retain control of their ability to sell and or collect on these assets if need be. Should they lose this ability, they simultaneously lose their ability to effect the money supply on the downside and inflationary risks are increased all else equal.
Over the last year the Feds' balance sheet has undergone quite a makeover (see image below) yet the value quantities of assets hasn't changed much. One reason this is more serious than the money base aggregates suggest is because the Fed has been replacing higher quality assets with comparatively lower quality assets. The fed went from assets that have little no to risk of default before expiring and are generally very liquid to loans backed with questionable credibility and liquidity. This may then effect the Feds' ability to unload these assets at significant value if need be and consists of a debasement of the Fed's creditability. In all, this is one reason to why what the Fed holds matters very much so and explains a deeper picture than is captured in a simple aggregate.