The Silmarils

[Gwaihir the Windlord]

(that is, of course, until he lets much of his power pass into the physical world).

In which case he has converted his power into an invested form, which still makes him power-rich.

But it is true. Although Manwe was more akin to Illuvatar, Melkor was in the beginning created with powers that surpassed all others. He did fairly well in his various (primordial, War of the Powers) wars with the Valar, if you think about it, for saying that they were fourteen Valar onto him alone (with Maiar on both sides, but more on the side of the Valar). His influence on the world was huge, and that was impacted through his power.

I do not doubt that if it had been Melkor and Manwe alone, two singular opposing powers, Melkor would have prevailed. As it was, the Valar were many and united and stood as a more solid, formidable foe.

Having said this, there is another interesting possibility. Manwe, sitting atop Taniquetil, had more than one 'revelation' and once a very extensive (but still unsatisfactory) vision of Men, that were granted to him through Illuvatar. Could Manwe in fact have increased in power while within Arda, through Illuvatar? It is doubtful that Melkor would have recieved these things from Eru, as his mind was closed to his maker.
On second thought, though, these communications between Illuvatar and Manwe probably were no more than communication. Manwe was, after all, the most insightful of the Ainur into Eru's mind; it is likely that he was simply using this skill.