Rivendell always appeared more as a household to me, and not exactly having a permanent population (aside from Elrond, his children, and other members of his house). It was established as a refuge in the 2nd Age, so the population would have probably constantly fluctuated. And after Sauron's defeat in the the Last Alliance it became known as a house of lore, where Elrond (of course) dwelt and travellers went to seek council. The Council of Elrond might be a good judge of the regular business of Rivendell. A safe place, a refuge, but relatively few people living there permanently.
Where Lorien had been settled by Silvan Elves for ages, and it accepted Amroth, then Galadriel and Celeborn as Lorien's rulers. The population would have likely consistantly declined as the Elves continued to leave for the West or die in wars. But don't forget Lothlorien fought off 3 assaults out of the Misty Mountains and Celeborn led an army to aid Thranduil from the assaults out of Dol Guldur.
So, Lothlorien's population was permanent (and my guess is a steady decline). Where Rivendell's, from the fact that it was established as a refuge would have not had a permanent population except for Elrond's household.
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Fenris Penguin
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