Leader larger than main line or vice versa?

[Bream Master]

Just wondering about this and I can't remember why but this issue hit me in class yesterday.  I had a long complex question too but can't remember it know.  Maybe it'll come as you guys clarify.

Thanks guys.

[Cajun1977]

both          sometimes you need a more smaller line to fish a spot or situation       and sometimes you need some big line to fish cover and such        each situation is different theres no wrong or right in general           just situations