[Rumil]

As TheAzn says,

an equally powerful stone thrower in an elevated position could shoot further than one at ground level. However, this increase in range can never be more than half the range on the flat, assuming everything is shot at the most efficient 45 degree angle.

Reconstructed medieval designs for trebuchets can shoot a heavy projectile out to about 300 metres, while with modern materials and improvements trebuchets have been designed that can throw a pumpkin 620 metres.

The question remains as to why Sauron's catapults outranged Denethor's. Of course nobody knows the answer, but a possibilities suggests themselves. First of all the obvious conclusion is that Sauron's catapults were just more powerful, but why could't or didn't Minas Tirith make similarly powerful machines?

First, Sauron was a Maia originally of Aule! If anyone knew how to build a war machine, it was Sauron. Maybe Gondor's machines were just not as technically advanced?

Secondly, the medieval trebuchets were very big, typically they were transported in bits to a siege and constructed on site. It may just be that there wasn't sufficient space on Minas Tirith's ramparts and towers to build a huge enough example to outrange Sauron's.

Thirdly, the Gondorians seem to have considered the main use of catapults as battering down walls and were confident that their walls were un-batterable. So it might have made sense to them to build more smaller 'antipersonnel' catapults to take a toll of any force attempting to scale the walls or use siege towers etc. This of course leaves them open to counter-battery fire. But remember they didn't have Sauron's enormous resources, so may have had to settle for a less than ideal artillery arm.