If I'm not mistaken the velocity mini is only a couple mm's shorter than the full size. Other models have a much bigger difference in size between the full size and mini. Anyway, another difference I've noticed with small rda's is the smaller chamber produces a warmer vape than a larger chamber with the same build and power. So if you were using a mech you need a less aggressive build to get a warm, dense vape with a small atty. I use my derringer on a tube mech and the performance is fantastic. Full size rda's get a larger set of coils and put on a vw box so I can crank the power and properly heat the coils, juice and air. Just a few rambling thoughts. Happy hunting.