The Weimar Republic had debt obligations that it couldn't repay and so they printed money. Meanwhile the balance sheet of the Fed is just that. A balance sheet. The only way they can create inflation is by making the balance sheet bigger. As long as inflation is below the target they can just keep increasing the balance sheet. There is no obligation to decrease the balance sheet unless inflation is above the target.

Why are they even doing this in the first place? The Fed buys assets during deflation and sells assets during inflation. Buying a cheap asset (e.g. $50 for a share) with money created from thin air increases the money supply and over the long run increases inflation. Inflation causes the prices of cheap assets to rise above the original value to $100 for a share. The situation is out of control! What can the fed do? It can sell assets in exchange for $100. In other words. The fed never runs into a situation which it cannot undo.