fastflyer is correct. The heavy designation is generally assigned to aircraft with gross weights in excess of 300,000 lbs. That includes the wide-bodies, and excludes the narrow-bodies. The infamous exception is the 757 which, due to the wingtip design, creates very tightly-wound tip vortices. The 757 trailing vortex system has been accused/suspected of contributing to fatal crashes of several trailing aircraft, and numerous incidents.