What a wonderful question. I just always accepted that it was what it was. Honestly the en and ed contractions at the beginnings of words were always difficult for me to understand, so I just memorized them and stopped attempting to make logic out of it. But since you asked, I did a little research and found something on merriam-webster.com that might answer your question. The origin of enormous: Latin enormis, from e, ex (meaning: out of) + norma (meaning rule). So perhaps e is a prefix after-all. And that is the best I have for you, I hope it helps.