"heavy" ball just means the person cannot hit center, cannot get a handle on the incoming ball with precision and comfort.
A "heavy" ball is different for different players. Old farts who played 35 years ago find heavy topspin feels heavy and mishits happen all the time.
Young guns might find the shin high slices with sidespin "heavy", because they don't regularly practice with old farts who slice and dice.
Weaker players find every shot "heavy" when they play better, stronger players.
Everyone finds Nadal's shots "heavy".