The heaviest ball I ever faced came from a German exchange student I played in the mid-90s. He used a Wilson Hammer 6.2 (skunk) racquet. The ball didn't look like much coming at me but it just pushed my racquet back everytime I made contact. If I didn't hit it perfectly in the center of the strings, I had trouble with it. I got to hit with this kid multiple times and the best I ever did was get 4 games in a set. He was easily able to play D-1 tennis. He grew up playing on clay courts and was just smooth on the ground. We only played on hard courts and if I slammed the ball as hard as I could, I could get winners but otherwise he'd just hit the ball back methodically. It didn't look like much at all but it was HEAVY and would twist my racquet on anything off-center.
I got to see him play one of the better hs players in the area as he wasn't eligible to play for the hs team. The other kids coach was watching the warm-up and saying he didn't think he was THAT good and his player would beat him. He just saw a kid who wasn't super quick and hit a decent ball. He couldn't see the weight behind those shots. The match started and the German easily won 6-0, 6-0. The opponent came off the court crying because he was humiliated by the fact he couldn't return the shots and match the placement. His power and hard-hitting game was no match for the placement and heavy balls of the German kid.
I honestly couldn't tell you what causes that heavy of a ball other than technique and timing. I can most certainly say that it isn't how hard you swing. It has a lot to do with the spin on the ball though. I do know the answer to playing that ball is to set-up early and to have your racquet prepared just as early to make returning that ball easier. Good luck.
Back to using POGs. Why did I ever leave you dear friend?