Jase touch on it pretty good but there's a bit more.
While aa and aaa have the same voltage, no load, the aa's will output more current because they have lower internal resistance. This is why a couple of my red's have no resistor using aaa's. If I put in aa's ---- POOF --- behold a weak LED :-(.
When building RED's with DVD LD's, you need to "characterize" it and draw your series circuit including all resistances, voltage drops and cell impedances. Even then, there's a big GULP when you push the button. !!!
Mike