I vaguely recall some 15 years or so ago the FAA got a bunch of complaints of turbulence from planes following behind a 757. They decided the 757 wing generated stronger than "normal" wingtip vorticies than would be expected for the weight. ATC was instructed to compensate by treating all 757's as heavy even if they wouldn't otherwise require the designation.
Memory is vague as it really didn't affect me one way or the other.
It boils down to this. All
753's are heavies. Some
752's are heavies due to extra fuel tanks and being rated for a higher MTOW then most 752's. Even the 752's that are not
heavies have special separation rules that apply to them on final, but it's not as simple as treating them as heavies.