I'd argue a bit with the belief that higher voltage directly translates into more heat. As you add more voltage with the same resistance, the current will increase, but in practice, most people use lower res with lower voltage. What this is doing is regulating the wattage. 6v @3Ω, 5v @2Ω, & 4.2v @1.5Ω should all be roughly the same wattage, but more and more current. Current, not voltage, produces heat.
I would guess that a lot of people try LR cartos/atties on some small battery like an ego which is incapable of outputting as much current as is required. The current caps at ~1amp, which drops the voltage down into the 2's. Then, switching to 6v with decent batteries, it runs hotter. A VV mod with a good battery that can handle the current draw of either setup should run cooler at higher voltage with higher res.