How much outside air actually moves through it
A typical residential outdoor unit (around 3 tons / 10 kW of heating capacity) is sized to move roughly 350–400 CFM per ton across its fan and coil — about 1,000–1,500 CFM (1,700–2,550 m³/h) for that size of unit, per standard HVAC airflow sizing. That’s several living rooms’ worth of air, every single hour, just to have enough of it passing the coil to pull heat from.
How much heat is actually in that air
How big the heat exchanger has to be
The coil’s visible frontal area is modest — around 0.5–0.8 m², roughly the grille on the unit itself, based on typical outdoor unit dimensions. But the thin metal fins packed into that frontal area multiply the actual air-contact surface roughly 15–20×, a standard extended-surface design used across finned-coil heat exchangers — pushing the real heat-transfer area to somewhere around 8–15 m², the floor of a small bedroom, folded into a box you could carry. This is an order-of-magnitude estimate, not a manufacturer spec: exact fin density and tube layout vary by model.