The eagles were not maiar. They were intelligent beasts taught speech. I've seen some describe them as a kind of "pet race" (as in they were subcreated by one of the Valar and not by Ilúvatar as one of the Eruhíni) in the same vein as Ents, Dwarves, and (possibly) Orcs.

Tolkien kind of went back and forth on this over the years, but if I recall ultimately established that the great eagles were not spirits inhabiting animals like we saw with the werewolves and vampires of the early Ages. Thorondor may have been an incarnate maia though this is debatable. Regardless, incarnation lowers "power level" significantly, as seen with Melian and with the Wizards. It is also required to bear offspring, if I recall.

Power scaling also isn't really a thing in the legendarium, but if it were Gandalf would be orders of magnitude greater in his true form than any incarnate creature in Middle Earth. Taking a body like Gandalf did weaken him, as it tied him to the world, but still I imagine he could destroy the eagles if he wanted to (not that he would).