I believe, and sit to be corrected, that the "problem" 757 was to do with ultra quick certification using 73 data in the process.
Certainly that`s what l was told after rolling through sixty degrees against full control deflection in a SF340, with 4 mile spacing on approach. Local conditions were a calm summer evening in otherwise still air and vis that went forever.
The term "heavy" for the 75 came in soon after, possibly the timing of this was coincidental.
If l remember correctly prior to 1993 it was certificated as medium.
I know some had big fuel tanks and plainly the induced drag between models varied.
Wing tip vortices is the problem on take off but with a "heavy" it can be one on landing too despite relatively low fuel weight.
`spose it`s best to know what`s in front regardless.
Having to keep saying "heavy" between atc units? Sounds good to me.