In the case of the Eurosystem, the main policy-relevant balance sheet component and also the one essentially driving the size of the balance sheet are the regular refinancing operations, where banks receive liquidity against eligible securities. Before the crisis about two-thirds of this liquidity was provided for one week, while the remainder for three months.
Unlike in the US, the financing of the real economy in the euro area relies extensively on banks. During the crisis the main policy concern in the euro area was therefore to ensure that banks would pass on lower policy rates to their customers and remained able and willing to provide loans to companies and households.