“a big fish in a small pond”是一句俗语,比喻为小地方的大人物.英文释义为"one of the most important people in a small group or organization, who would have much less power and importance if they were part of a larger group or organization"（一个小团体或组织中最重要的人物,如果到一个大一些的团体或组织中,其力量和地位将会大大削弱）.
"a small fish in a big pond"应该是由此俗语引申而来,比喻大环境中的小人物.把上面那个释义改写一下,可以这样说："one of the less important people in a large group or organization, who would have much more power and importance if they were part of a smaller group or organization"（一个大团体或组织中不太重要的人物,如果到一个小一些的团体或组织中,其力量和地位将会大大增强）.
e.g. I'd rather be a big fish in a small pond than a small fish in a big pond. 宁为鸡首,不为牛后.
P.S.有一个“大鱼小塘效应”（Big-fish–little-pond effect (BFLPE)）理论,由赫伯特·W·马什（Herbert W. Marsh）和马修·加德纳（Matthew Gardner）提出：学生的自我意识与学校其他同龄人的能力呈负相关的关系（the self-concept of students is negatively correlated with the ability of their peers in school）,即同龄人的能力水平越高,学生的自我意识越差.