The amount of heat produced by the arc is determined by the amperage. The higher the amperage setting the higher the heat produced by the welding arc, and the lower the amperage setting the lower the heat produced.
Each diameter of the electrode has a recommended minimum and maximum amperage range and therefore a recommended heat range. If you were to try to put too many amps through a small diameter electrode it will overheat and could even melt during weld and potential of the welder injury. If the amperage setting is too low for an electrode diameter, the end of the electrode may not melt evenly, if at all (Reason of Poor Start/ Stop defect, Arc strike)
Not all of the heat produced by an arc reaches the weld. Some of the heat is radiated away in the form of light and heatwaves. Some additional heat is carried away with the hot gases (Shelf Shielding Gas) formed by the electrode covering.