I think Kronos is the correct answer. Pop has gone from 1.5k to many many thousands (they won't release figures).
I only joined Nostalrius near the end.. but managed to hit level 20 there.
Kronos feels identical to leveling on Nostalrius, except without the server lag. Barrens is absolutely swamped with players. Chat is identical.
Main difference is that the cities / high level zones are much quieter, as the vast majority of the population is under 30.