good job getting it backwards, in the 80's mainstream bands were "metal" and the heavier bands were heavy metal and then there was Thrash [twopalmssquarelyonforehead]
"Metal" is obviously a shortened version of "Heavy Metal", so I consider them synonymous, but since "Heavy Metal" has come to take on connotations of what could also be called "Traditional Metal" or "Classic Metal" it is no longer convenient or perhaps even coherent to refer to non-"classic" subgenres by the original term.
I suppose the same thing happened to "Rock 'n' Roll", where the original term now refers primarily to the originators of rock music and one must refer to prog/folk/jazz/etc. rock bands as simply "Rock" in order to be understood.
Yeah, to me "heavy metal" is a specific sub-genre while saying only "metal" incudes all kind of metal music. And since I do not only like "heavy metal" I usually say only "metal" and included in that is everything from Morbid Angel and Anaal Nathrakh to Megadeth and Therion.