barbarian factions can't build large cities?

[craziii]

I am playing a few campaigns atm, but lusotana can't build beyond minor cities. it stops at 12000 pop, doesn't go any higher. I just looked through the building tree for it, and there is nothing after minor city lvl. why is this so?

just checked, but it seems like all barbarian factions can't do it. I was gonna try to play gauls next, but how are th gauls suppose to satisfy the huge city requirements for the reform? I mean it requires large cities no?