These army sizes are not one or two clans, they're the combination of 20+ tribes and clans - and that only got them to 40,000 - only when a whole other confederation joined with 10,000 did the number go to 50,000

We need to keep in mind that the Golden Plains are gigantic - it's only natural that there would be huge populations there. Huge plains, lots of herding grounds, bigger herds, they can maintain bigger tribes, and so on. Wars diminish their size from time to time, but they breed like rabbits.

And plus, I believe we have naturally occurring metals that are harder than bronze - the swords and other weapons Rukbans use come via merchants from the tedar and whatnot - and perhaps there is the odd blacksmith who has learned. So we're not actually in the 'bronze' age on that front.

I definitely think that somewhere like Alefpria would have populations north of 100,000

Edit: plus, steppe hordes whether in 2000 BC, 100 AD, or 1200 AD lived in pretty much the same way. If Attila and the Mongols could muster such large forces, any significantly large steppe culture should likewise be able to. It's not like the Mongols had medicine and technology which was far greater than that of the Huns