Daedric planes are a helluva lot less simple than Mundus. It's like the difference between a detailed portrait of a human face, and a stickman. One takes a lot less effort than the other.
But we are still comparing infinites, right? It may be a portrait vs. a stickman, but it's an infinite portrait vs. an infinite stickman. And, couldn't it be that planes of the Daedra, created by each his own, took as much from each Daedra as was taken from the Aedra in the joint creation of the Mundus? Since Nirn is more complex than Oblivion, a single Aedra (or Daedra, for that matter) couldn't have created it. But together, the Aedra were able to form an infinite with complexity by expending the same "energy" as a single Daedra in the formation of the simpler, but still infinite, Oblivion? Of course this all relies, as you pointed out, on the notion of "creation." If the Daedra created nothing, and the planes
are the Princes, then this theory doesn't hold up.
But there is another: the Dawn Era, being without linear time, cannot be defined by the thermodynamic arrow of time (or Akatosh's arrow, maybe). Which means that the presence of Akatosh during the creation and the "trap" cannot be pinned down. The only thing that can be placed in a mortal context would be the creation of the Adamantine Tower. So it is entirely possible that Akatosh simply wasn't effected by the creation of the Mundus in the way that the other Aedra were. His role as time itself puts him outside of the realm of events until he chooses to solidify time. So perhaps Akatosh would be the
only Aedra who could take on Dagon (hey, that was a poem! It was a stupid poem, but it was a poem nonetheless).