Efficiency of a speaker is much more than the size of the coil. Suspension stiffness can effect it. Weight of the cone and cap can effect it. Gap etc. etc.
Well I had 2 JL 12's on a 1700W amp and 4 Gauge wiring, but I wanted something even louder, so I bought the SA-18 and a Rockville 6000W Amp and I'm running it on 4 Gauge wiring again, just looking for much louder bass.