Big clubs in the EPL is a more global thing I think. A big club there is one with a big global following.

Below the EPL you then look at; stadium size, season ticket holders, and historic acievements (doesn't necesarilly mean silverware but seasons in the top flight etc).

It's also relative isn't it. Are Bradford a big club? Probably not. Are they a big club for league 2 - almost certainly. Same for Derby, Bolton and Portsmouth in league 1 and Oldham in the Conference.