The mass of the cone will drastically affect efficiency. Tripling the cone mass reduces the acoustic output by one half. Magnet size is also a key factor. Theoretically, doubling the magnet size will double the efficiency. The cross-sectional shape of the wire used on the voice coil also plays an important role.
Power handling is also an important criterion when selecting a driver.
Be careful when comparing this rating. Peak-power ratings are usually double the rms rating. The difference in these ratings is easily understood. The electrical power-handling capability of a speaker is directly related to the voice coil's ability to dissipate heat. Large voice coils have more surface area and are able to dissipate heat better than small voice coils.