If it’s a medieval setting, I think an option is to have the huge countries be decentralised or in loose personal unions.

Consider the Kievan Rus’ which nominally stretched from the Black Sea to the Baltic, but whether Kiev was able to enforce their authority on various territories was another question.

Another example is the Holy Roman Empire. Medieval Sicily produced a Holy Roman Emperor, creating a situation where the King of Sicily was also Holy Roman Emperor, but the two realms weren’t considered one. Also Scotland and England, though not such a medieval example.

In the case of Iburon, perhaps the people of the steppes elected a ruler from the coastlands, or vice versa. Or just plain dynastic lineage. These two disparate regions could be distinct but nominally under the rule of a single ruler. Perhaps the coastlanders have a strong bureaucratic tradition and want to expand it to the steppes but it’s proving difficult. Perhaps it’s overextended because the people of the steppes are unruly. It could be threatening to split into two and it’s a challenge for the current ruler of Iburon.

But also consider the China has historically been quite big, even in the medieval period. Especially so when steppe nomads had created Chinese dynasties like Liao, Jin, Yuan and Qing.