I think the issue is, the Blood Elves HAVE lost far more people than High Elves.
Firstly, of the survivors of Quel’Thalas from the Third War, 10% would retake the name High Elves, be exiled to Quel’Lithien in the Eastern Plaguelands, and then largely become Wretched.
So, Blood Elves’ original numbers are now down to 90%.
But wait, there’s more!
Kael’thas took the strongest 15% of the survivors as his force when he went to seek aid from the Alliance. This number would be subjected to suicide missions by Garithos, the Outland campaign in which Blood Elves were literally throwing themselves into the meat grinder of the Black Temple’s front gates, and then another campaign by Illidan to Northrend, which was ultimately lost. So, three campaigns later, and what happens? The Scryers defect, which is noted as the largest loss of manpower Kael’thas ever suffered. The rest? Suffering another failed campaign in Outland as Kael’thas joins the Legion.
Let’s be generous and say out of the 15% Keal’thas took after the Third War, 5% of that returned to rejoin Quel’Thalas.
The Blood Elves are now at 80% of the survivals from the Third War. Or, more likely, closer to 75% considering the playable Blood Elves were involved in fighting for the Horde and Kael’thas stormed through Silvermoon to grab M’uru.
So we’re down to 75% of the survivors of the Third War.
What next? The Northrend Conflict arrives! The Blood Elves are forced into it thanks to Sylvanas, while still recovering from a costly Civil War with Kael’thas. Worse, Aethas is taking a number who become the Sunreavers and decide working with the Humans, Dwarves, and Gnomes is so much better than living in Quel’Thalas.
Let’s be generous again and say this is a 5% loss, which brings us down to 70% of the survivors of the Third War.
So next comes the Cataclysm and Garrosh’s reign as Warchief, depleting the defenses of Quel’Thalas to the point that when the Amani resurface, Halduron has to call on High Elves for help. I’m going to err on the side of generosity and say there was no significant loss during Cataclysm.
So, still sitting at about 70%.
Mists of Pandaria comes around and Lor’themar is reconsidering joining the Alliance because Garrosh has been throwing Blood Elf lives away as fodder for his war, suggesting casualties were unacceptable. Lor’themar goes off on his own to the Isle of Thunder, and is present for the Siege of Suramar during which there are casualties since Sylvanas offers to raise the fallen Blood Elves.
Going to say 10% probably lost here, since Lor’themar would have had to of been pushed hard to reconsider the Alliance. So, the Blood Elves are now down to 60% of the survivors of the Third War.
We move on to Warlords of Draenor where Liadrin leads Blood Knights to Draenor to support the Draenei. I’m going to be generous and say there aren’t many casualties here, since nothing drastic was noted.
Move on to Legion where the Blood Elves are aiding multiple class hall orders. Again, no major casualties noted, so I’m not going to say any where lost.
Now we’re at Battle for Azeroth. Nothing yet to suggest a lot of Blood Elves are being lost, but we know there was a, ‘suspiciously large amount,’ of them in Dazar’alor, meaning they’d have been present during the Alliance’s attack. Liadrin led forces in the Battle for Stromgarde, and we saw one in the Darkshore Trailer suggesting that there are some out there getting mauled by the Night Elves as well.
Depending on where BFA goes, the Blood Elves could be dropping another 10%, estimated.
In other words, by BFA’s ending, being generous in population estimations, the Blood Elves will have depleted nearly half of their numbers from after the Third War. This doesn’t even take into account the loss of the Void Elves, or the fact more Blood Elves may be defecting and becoming Void Elves. The Blood Elves could well be below 50% of the survivors of the Third War before this expansion is over.
TL;DR - Population numbers are nonsense, as right now there shouldn’t be enough Horde and Alliance left to fight over so much as a block of cheese as a war.