The whole Roman naming system was kind of in shambles at that point to be honest (and let's face it, had been ever since "Flavius" became a first name and personal names became determined by cognomina roughly around the time of Constantine I), and I for one honestly can't accept that Magnus Maximus's parents literally named him "Greatest of the great". Seeing as history only records Flavius Victor as Maximus' son, "Flavius Claudius Constaninus" could just be a name the guy adopted.
As to how "Aurelianus" arose in the first place, I'm going with the adoption theory (ergo, someone in the gens Artorius adopted a guy citizen'd by the Antonine Edict), at least back in the day when the naming system was more solid.
Magnus Maximus is exceedingly unlikely to be the guy's birth name. So you can give him any birth name you like.