A person can get the same "amount " of work by using less weight as well. As long as the session is dictated by form and consistency, muscle fibers can be torn and broken down just as effectively as "goin heavy". That is why it is best to incorporate both.
Strictly lifting heavy will target mainly your slow twitch muscles, neglecting fast twitch fibers. In essence, if you only do one or the other, you are doing yourself a disservice. To get strong most effectively, train with light AND heavy weights.
Crossfit.com <--- no wimps allowed