[Legate of Amon Lanc]

Originally Posted by Lalaith

Yes, absolutely, I think that's a very interesting point. Did Gandalf's corporeal form, as an old man, lay "heavier" on him than Melian's, do you think, despite the fact that she bore a child while a physical, Middle-Earth presence?

Well, I am not a female, so I cannot judge whether it's more exhausting for the body to give a birth to a child or not to, but be much more aged.

Nevertheless, Melian, I believe, had her "Valinorean" form on her (cf. below) when she met Elwë(is this where your train of thoughts has been going?), so maybe here is the answer to what you wondered about her powers like the Girdle and so on. Also, she was not intentionally bound by the body - that was her own body as she chose it (unlike the Istari, who were given it to reduce their own powers not to contest Sauron by force) or even the Balrogs, who were in fact "forced to hold Melkor's standards", so to say.

Originally Posted by Lalaith

The Valar and the Maia took physical forms while in Valinor, also. Were these forms somehow different to those they took when not there? Is there anything in HoME about this?

As I foreshadowed earlier, I believe that surely they were different; as mentioned above at least with the Istari who took the forms of old men for their journey to Middle-Earth, but surely were not like that back there. The difference would be that probably mostly, in ME the Maiar were limited in their forms - in Valinor they could take on whatever form they wish (the one they chose when coming down to the world). The "dark" Maiar, like the Balrogs, or even Sauron later, as it is well known after some time lost their ability to change their forms and literally degraded as "imprisoned" in the flesh.

I'm not much of a HoME-runner, so to say, but considering the appearance in Valinor, at least I remember in UT Gandalf (Olórin) is mentioned:

Originally Posted by Istari

And Olórin, who was clad in grey, and having just entered from a journey had seated himself at the edge of the council

and in the Silmarillion, resp. Valaquenta:

Originally Posted by Valaquenta

But of Olórin that tale does not speak; for though he loved the Elves, he walked among them unseen, or in form as one of them, and they did not know whence came the fair visions or the promptings of wisdom that he put into their hearts. In later days he was the friend of all the Children of Ilúvatar, and took pity on their sorrows; and those who listened to him awoke from despair and put away the imaginations of darkness.

(emhasise mine) Taking in mind the context of the latter, I'd say the form of Elf refers to Olórin during his stay at Valinor, and not Middle-Earth. The interesting thing is that he was "clad in grey", still, on both sides of the Sea. I'd also like to see what happened when he returned at the end of the Third Age: if he kept his form of Gandalf the White, or changed into something else (Olórin the Gray, Olórin the White...).