I still think you're thinking of the wrong kind of "epic" imfletcher, Possum is correct, it's to do with story. We're talking about 'Epic' as a
Genre, Not in terms of subjective qualities.
For instance, in terms of Epics; Jason and the Argonauts could be considered an Epic, as it's the story of a hero, same with something like Troy, LOTR etc. But film's that "make it big", or have a cult status, or are just "epically" good, doesn't make it (in genre terms) and Epic, even if it is.. epic.