The title might be not so clear, so I will try to explain myself the best I can:
Nowadays, historians have formal ways to define what an empire is. For example, wikipedia says the following: “An empire is an aggregate of many separate states or territories under a supreme ruler or oligarchy.”
Because of that, many entities are called nowadays empires by historians even if during their time their rulers never called it that way nor were they called emperors.
Some emperors of the past would get that title from the pope, like Charlemagne or Napoleon, in two epochs when many other rulers in Europe just called themself Kings.
Other entities would be called empires just by themselves, like Mexico, Brasil or Haiti, even if Spain, Portugal or France were not called empires at the time they had those territories.
Then, my question is, what avoid rulers from middle age to say “I have a pretty big territory, I should stop being called king and start being called an emperor”?
To put a few examples:
I mean, being an emperor was the biggest distinction in the western world. Seems like everyone wanted to be like the roman emperors, then why was that title so uncommon?
Thank you and I hope I explained myself clearly. I’m really confused about this topic and I think I lack some important information to understand it.
Traditionally catholic monarchs would not call themselves emperors, they'd leave that to the holy roman emperor which was "technically" the main authority on catholic affairs after the pope. The papacy wanted an united catholic empire in europe led by one emperor, having multiple ones would destabilize the authority of the HRE. The pope only crowned the holy roman emperor, kings were crowned by archbishops. Protestant and orthodox kings were free to call themselves emperors and they did so. For example, Russian monarchs started calling themselves Tsars after Ivan III married the niece of the last byzantine emperor and so they claimed to be the successors of the roman/byzantine empire. The british had all the opportunity to do so but the monarchs rejected to call themselves emperors, only adopting the title when they got India. The german empire was formed to consolidate power over the kingdoms in the german territory, there was a need to have an higher title.
With the independence of brazil, the ruler of brazil had the opportunity to call himself emperor because he had no territory in europe and had no direct control from the papacy, also, he wanted to emulate Napoleon. His father who was the king of portugal and who was still alive was emperor of brazil for a few months and his son inherited that title.
There were some monarchs in iberia who called themselves emperors of all of spain, this was a tradition that occured before the rise of the papacy, it was a time when the catholic church was heavily weakened, pre-crusades. Calling themselves emperors of all of spain also meant they claimed all of iberia, giving them casus belli on all their claims in that territory. This title was lost to the wind when multiple kingdoms started to form in Iberia and all of them recognized the emperor of the HRE. During the Iberian union there was an opportunity for Phillip II to call himself emperor but naturally it would have collided with the Habsburg monarchy of austria who controlled the HRE and they were allies that defended each other from french and british aggression.
This did not stop people from calling this kingdoms empires and their monarchs emperors, it was a contemporary tradition that was not censored and was not unusual. In practical terms they were empires.