What you're describing there applies to a good old-fashioned series rheostat dimmer, but triac units don't work quite the same way.
With the rheostat, as you insert resistance to dim the light you get heat dissipated by it due to I^2*R.
With a triac unit, the RC dimmer control determines at which point in each half cycle the triac is triggered into conduction. Before that triggering point, the current is practically nil (just the tiny leakage through the PN junctions).
There's a small amount of voltage drop across the triac when it's conducting. As you increase the brightness of the lamp, the triac is conducting for a greater portion of each half cycle, therefore it gets hotter.
