I've had a theory for a while, and others' reports whom I've asked seem to pan it out, is that XP gained in Besieged is based on your job and your actions. For example, a WHM curing and raising others would get proportionately more XP for the same actions than, say, a MNK/WHM. PLDs do seem to get XP for taking damage, as well, whereas other melees I've spoken with don't seem to .
It's hard to test though when making controlled situations involves asking others' to sacrifice the possibility of full besieged XP.
