The Ainur and their physical forms

[Neithan]

The potency of their spirit then translates to the strength of their corporeal manifestation.

I had assumed this was the case as well, until this quote came to my attention.

Greatest in strength and deeds of prowess is Tulkas ... for he can outrun all things that go on feet, and he is tireless.

Tulkas is clearly portrayed as having the greatest physical prowess, but when Tolkien lists the Valar "in due order" he puts Tulkas last.
In fact as this quote is what prompted me to start this thread in the first place I probably should have posted it earlier.

I'm actually having a bit of trouble answering your questions on this because I do not have access to my books. I am rusty, and cannot recall where Melkor's capture falls in the timeline. I am tempted, though, to say that Melkor had already been significantly weakened in spirit (which was directly connected to his increasing incarnation), and was perhaps now not so impervious as he once was.

He was captured the first time just after the Elves awakened. After he was released he could still assume fair form, and I think he still had most of his power. At any rate he was definately not incarnate yet. And he was still referred to as "the mightest of all the dwellers in Eä" well after this happened.