Baditude would be the guy to explain step down clearly, but basically if you have a 4.2v battery and you are trying to deliver anything lower than that, devices without step down will still send 4.2v.
With the build you described at .8Ω, firing it at 16 watts means you WANTED to send 3.6(ish)v to the coil, but without step down, you were still sending 4.2v (22 watts),