I was going to SAY! That sounds like a reasonable scale.

Many of the dead in even a world war would be a.) soldiers and b.) some targeted cities. It makes no sense to kill off half of the planet, because most wars involve land AND population conquest and having subjects under your control if you win is part of the point, too. Piles of ash don't pay taxes/tribute or produce anything.

A number being a large amount is not enough. You need to put it in the proper context. Six hundred million is a large number, every death a tragedy etc, but in the context of destroying the whole pre-war paradigm, rendering the concept of the nationstate useless, requiring the literal intervention of aliens to fix, etc, it is not large enough. It is a recoverable number, especially as it is canonically said, by a main character no-less, to be the total Global death toll. That is a recoverable number, even today. That would not destroy the whole pre war order.

A more reasonable figure, every death a tragedy etc, would be two-three billion. A third of the the world. Even then, that is not a complete game changer depending on how evenly spread the destruction was.

Ironically, the franchise that gets this right is Neon Genesis Evangelion. They killed Three Billion in the backstory, and made a massive chunk of the world uninhabitable. Yet the world continued, a bit crappier, and with harder work to eke out a first world living in, but it was doable. And that was when we only had six billion on the planet. They killed half the world, including a lot of first world nations, and civilisation went on nonetheless.

A large number is not enough. It has to be large enough in context. Six hundred million in context of the changes it wrought and the population it started with, is just not large enough. YOU have no sense of scale.