The reasons why Charles was nicknamed the Great?

First, we can say that he looked “great”: he lived 70 years (quite a lot by those standards), of which 46 years he ruled. He was 192 cm tall and had a large mustache.
Secondly: he studied several foreign languages at once, understanding each one as easily as in his native language. I also read many books.
Thirdly: I went on military campaigns many times; for example: against the Longobards in Italy, in Spain against the Arabs, against the Saxons.