The Engine mini is a slimmed down version of the original, with a dual coil deck and I don't remember, 3 ml capacity. The Engines have been very successful tanks thanks to their high leak resistance. I love my full size Engines and have them in daily rotation. there is an older thread about the dual coil Engine in the RTA section
OBS Engine RTA... or is it an RDTA?
Much of what is discussed applies to both versions. There is also an Engine nano which is a single coil deck that also has it's own thread. The nano has had a mixed bag of attention with performance appearing to some users as less than the originals.