I guess it's a lot since it's 2 of the 9 classes, but that still means i face the other classes more often. I don't really mind. Your class in arena doesn't determine how many wins you're going to get. It's possible to do well with all of them and my winrates are about teh same against every class.
But you are a very limited sample size. Plus, the other classes are front-weighted. You will see a decent mix early on, but once you get to 6 or 7 wins, it's Mage and Paladin city. Other classes can get there, of course, but that usually means they had an _insane_draft.
What makes Mage (and Pally) so effective in Arena is that their most powerful cards are Commons: Flamestrike, Fireball, Polymorph, Water Elemental, Snowchugger etc. Because of this, they get them much more often than other classes get their equivalents.
Plus, both hero powers are insanely good in the Arena format.