When Mexico became independent the plan was for the country to become a commonwealth with Spain, the king serving simultaneously as the Emperor of Mexico. This did not work out and Augustin Iturbide became emperor instead. In Brazil it was the son of Portugal's king that proclaimed himself Pedro I, Emperor of Brazil. Why did these countries opt for emperors rather than kings? Was their any precedent to this? Or was it simply a matter of outdoing the former colonial power?
By the time Mexico and Brazil became independent nations in 1820-21, the title "Emperor" was being tossed around pretty casually by the European powers. Napoleon had just been Emperor, the Austrians had an Emperor, the Russians had an Emperor. Eventually there would be a German Emperor and the British used the title of "Emperor of India". My guess would be that using the title of Emperor in Brazil and Mexico was an attempt to assert equal footing. Though you do make an interesting point, the Kings of Spain and Portugal never took on the titles of Emperor. I don't know of any primary sources on the topic, so I'm trying to make an inference based on the usage of the title at the time.
Im spanish historian so i only know well about México.
Seems to me that you only can became a King if you are ruling country whit feudal system. Both cases happens in post colonial era and Spain never implemented a feudal system in America. Second to become a king you need to have "blue blood" and be part of a dinasty.
Both Brasil and México had enorumus land compared to iberian peninsula. So they are seen as empires like asian Empires.