If I'm not mistaken than heavy ball can have 2 meanings. One describing the air pressure or weight of the balls and another meaning a ball with pace and spin. Momo is a good example of someone who hits a heavy ball. The heavy condidtions thing is just as you thought.
Heavy conditions - ball travels slower through the air and feels heavier...this is often caused by dampness.
Heavy ball - a ball with lots of "work" ( heavy topspin).....Knizzle uses MAURESMO as an example... which is correct amongst the women.
Capriati hits a HARD ball, MAURESMO hits a HEAVY ball.
Momo hits a "heavy" ball. Not necessarily with as much sheer pace though the air as some of the others, but with so much spin that the ball is difficult to return, as if it had more momentum than its pace would suggest, i.e. as if it weighed more than usual.
The men hit much "heavier" than the women, generally speaking, even if the ball travels no faster through the air. Momo is notable as a woman who does this.
In "heavy" conditions, the ball seems to be heavier, i.e. it is harder to generate pace on it.
(At least that's what I always thought these terms meant. )