Hey guys, so I was wondering.. Charlemagne got to be the Emperor because he was crowned by the Pope. The same with the Emperors from the Habsburg-Dynasty. But, for example Prussia got to be a kingdom because the Emperor said so, and the Pope didn't approve that. Napoleon just crowned himself to the Emperor of France and Russia was also viewed as an Empire but since it was Orthodox it will not be the Pope who crowned him. On the other hand, the UK owned 1/4 of the world and always stayed a kingdom, not an Empire. And when Germany was proclaimed, it was a big dispute whether Wilhelm I. could call himself an Emperor or not. How was it decided back then what was a kingom, what was an Empire or a Duchy? Could you just go around "Hey this guys looks mighty, I guess he could make me a king"?
The general rule is that a King rules or reigns over a kingdom, whilst an Emperor rules or reigns over multiple ex-kingdoms that form an empire.
But this is a very general rule and the only way to answer is to look at the significance of the titles in each situation. The United Kingdom, for example, remained a Kingdom because her colonies and territories were separate entities.
As the empire wasn't a country in it's own right, it was more akin to a group of independent kingdoms sharing a Queen. That was, until Queen Victoria's daughter married the Emperor of Germany that she instituted herself as Empress of India in order to avoid bowing to her.
On the other hand, the German Empire was a series of nations that were conquered and became one empire. The German Empire was a country, whilst the British Empire was not.
Sprouting a third hand, there have been empires like the ancient Empire of Carthage that were one singular country that didn't conquer any others, and the title of Emperor was more fitting as a translation than King.