Cardiac tamponade is a pressure problem.
The danger is not determined only by the size of the pericardial effusion. It is determined by how quickly fluid accumulates and how much the pericardium can stretch.
The pericardium is relatively stiff. If fluid accumulates slowly, the pericardium can stretch over time and accommodate a large volume.
If fluid accumulates rapidly, even a small volume can sharply increase pericardial pressure.
Once pericardial pressure approaches intracardiac filling pressures, the heart cannot fill normally.