Saying that, Myths Transformed XI p426-427 goes on to speak of everything native to Aman having a rate of growth equal to one Valian Year = one Middle-Earth year. Because of their "immortality", all things in Middle-Earth were swift to change and die or pass away. Etc etc, I doubt I need to quote it to you. Anyways, it was because of that part that my take was of no real change as far as physical growth is concerned, only in mind.
But wouldn't that mean that Elves grew or grew up far swifter than plants and animals in the world of Aman? For example if an Elf-child had a puppy, in about 50 years the average Elf would be an adult, but the Aman-puppy would still be less than one year old. Not impossible of course, but I think the growth of Elvish bodies is included. I also tend to think Tolkien later abandoned this concept, and had his Elves age at about the same rate as Men... unless that too was the ultimate result of a dwindling in Middle-earth! Which doesn't seem all that likely to me, but...
That's true, and I realize one has to start somewhere
I did read that the Cuivienyarna is fairy-tale / lore, but if we can't assume it as "fact", at least for this thread, then there's not much to build on!
Do you know roughly where I can read about the later revision of "re-birth", before I search for it myself (though, the separate Index book makes finding things pretty easy)? Still, for the purposes of figuring out population, I don't think it's very relevant during the First Age and before.
This road leads to the Glorfindel essays (Glorfindel I and II) in The Peoples of Middle-Earth. As other roads do too
'(...) Whereas on Earth to them all things in comparison with themselves were fleeting, swift to change and die or pass away, in Aman they endured and did not so soon cheat love with their mortality. On Earth while an Elf-child did but grow to be a man or woman, in some 3000 years, forests would rise and fall, and all the face of the land would change, while birds and flowers innumerable would be born and die in loar upon loar under the wheeling Sun.'
Text XI, Morgoth's Ring, Myths Transformed
Three thousand years! yet I think this might reflect an earlier (internally earlier) maturity rate for Elves, with the rate becoming swifter in Middle-earth. In the same text, it is also said: 'Nonetheless the Eldar 'aged' at the same speed in Aman as they had done in their beginning upon Middle-earth' And Christopher Tolkien noted in his commentary on Text XI: 'I realized that it stands in fact in very close relationship to the manuscript of Athrabeth Finrod ah Andreth,...' and in that text Finrod notes:
'This I can well believe,' said Finrod: 'That your bodies suffer in some measure the malice of Melkor. For you live in Arda Marred, as do we, and all the matter of Arda is tainted by him, before ye or we came forth and drew our hroar and their sustenance therefrom: all save only Aman before he came there. For know it is not otherwise with the Quendi themselves: their health and stature is diminished. Already those of us who dwell in Middle-earth, and even we who have returned to it, find that the change* [*the word change was an emendation to the typescript B (only); the manuscript has growth -- footnote by CJRT] of their bodies is swifter than in the beginning. And that, I judge, must forebode that they will prove less strong to last than they were designed to be, though this may not be clearly revealed for many long years.'
Athrabeth Finrod ah Andreth (and see Author's note 7 on the Commentary)
This work is generally dated to about the same time (late 1950s) as Laws And Customs too -- noting that the manuscript of the debate is said to be: 'very similar in style and appearance to that of Laws and Customs among the Eldar.'