I just may figured out how big magic source Xerath is
In Galio, he says Lux shines like a star. But when she started to lose her magic due to Galio, he points she doesn't shine anymore.
So, magic can be seen by magical beings but not mortal. Can that be because mortals are not sensetive magic to enough? What If Xerath's arcane magic is so dense and high, It can be seen even by mortals?
In Bloodline, It is said, Xerath shines more than Sun Disc ever did.