My ten-year old air conditioner was called high-efficiency and I was told that was why the outdoor unit was so large. A neighbor replaced his air conditioner and his outdoor unit is about half the size of mine. Why is this?

Generally, higher efficiency units are larger than standard units. This is due to the added coil area for heat transfer. In addition, some manufacturers make units that are physically larger than other manufacturers. Some of this is due to their desire to make the unit quieter as well as more efficient. When a unit becomes larger, room becomes available to put the compressor in a sound insulated compartment instead of exposed in the fan compartment. It also permits the use of a larger fan blade spinning at lower speed reducing the sound level.