Controlled electromigration of gold nanowires of different cross-sectional areas to form nanogap junctions is studied using a feedback method. A linear correlation between the cross-sectional area of the gold nanowires and the power dissipated in the junction during electromigration is observed, indicating that the feedback mechanism operates primarily by controlling the temperature of the junction during electromigration. We also show that the role of the external feedback circuit is to prevent thermal runaway; minimization of series resistance allows controlled electromigration to a significant range of junction resistances with a simple voltage ramp.