Why filter area matters more than the MERV number
Air has to pass through the filter. Tighter media resists more airflow, and if you tighten the media without adding area, the system loses airflow. Lost airflow means lost capacity, higher power use, and in cooling mode a real risk of freezing the evaporator coil.
Adding area solves the same problem from the other direction. A deeper cabinet with far more square inches of media can achieve high filtration efficiency at lower resistance than a thin filter in a small slot.