Ok, in 1990 Randy Barnes set the world shot put record by heaving the 20 pound ball 75 feet, 10.25 inches..
But in 1988 Szymon Ziolkowski from Poland heaved a hammer 83.38 meters (273.56 feet).
So if we assume these atheletes are around 200 lbs, are about 6 feet tall, and are throwing an item that weighs 1/10th of their weight we can see that there is a massive difference in the distances based almost entirely on how the item is thrown. Shot puts are thrown with the ball against your cheek and your arm cocked, with the force coming from legs, arm, and fingers.. The hammer is gripped by a handle and swung around and released at the highest part of its arc..
What this means is that if a man just chunks 1/20th of his weight he can make it travel about 12.5 times his height. If he grabs it, swings it around, and FLINGS it he can make it travel about 45 and a half times his height!!
So by this example a 30 foot tall monster weighing 2000 lbs could grab a human and shot put him 375 feet, but probably won't because shot puts are round but people are long, and it would take a couple of rounds to compress the hapless human into a nice round mass..
More likely he will grab the human by the feet and hammer-throw him, up to 1350 feet in this case.. 1350 FEET! This is just a height/distance equations all things being relative and not getting into mass, density or whether two African Swallows can carry a coconut by gripping it by the husk....
Any how, the people in these examples are world record holders and Olympic gold medal winners, so they really have the DND equivilant of having multiple feats in "Throwing Things REALLY Far" that not every Giant or Huge monster is going to have, so just being heaved 600 feet seems like a good compromise..
Oh, and we are forgetting that the thrown people will be traveling in an arc parallel to the ground, and when they hit they are not going to stop, they are going to bounce, roll, tumble, and get smeared all over the place, so you could just assume that the giant throws the hapless adventurer much less than 600 feet, but the remainder of the distance is him sliding face first through the gravel. (And briars, and thorns, and broken glass.)
CAM