What jarno said. A fighter->mage 7/1 is not a more powerful mage than a level 2 mage. In magical things, mind you. Which is what a familiar is about.
It's even more evident if you switch it around, a mage->thief 3/25 shouldn't have their familiar be ultra powerful as the mage is still a clown at magic.
@kreso: IIRC a familiar could pretty much be all of the above. It's either a "natural" creature that gets imbued with a smattering of magical power when it becomes a familiar (a cat, for example). You could also gate a small create and bind it to you (imp) or have a semi-mag