I'm looking for phrase/proverb about unimportant persons?

Question

The phrase I'm looking for has similar meaning to "in the land of the blind, the one-eyed man is king". When someone is important but the place that person is in not big or important, or when someone is around bad looking people so he/she feels/thinks good looking.
How can I say someone is good/beautiful/cool/important in a certain place, but if that person leaves that place this person is nothing? For example, a kid is cool in high school but when that kid goes to college there are thousands of other cool kids. Maybe it could be a insult to say to these person like "you are king of nothing".

Answer 1

You might be looking for "big fish in a small pond":

A situation in which one person has more power, influence, knowledge, or experience than others within a small group. It often implies that the person may not have as much clout in "a bigger pond," i.e., a larger group or arena of some kind.
_{from freedictionary's "idioms" section}