Resistance in the mod creates heat. The more resistance there is, the more heat that will be generated. It also depends on what kind of current is being pulled through the mod. So, in short, AMPS + Mod resistance = Heat. This is my best guess at what the problem is. The way to test this would be seeing how high the voltage drop is on the mod.High internal resistance leads to a larger voltage drop. My nemesis (clone) has .6 voltage drop on a 1 ohm coil. That heats very quickly at low ohms. My manhattan (clone) has .1 voltage drop on 1 ohm. This barely heats on low ohms.
Let me know if you have any more questions.