i use 20 PSI on all my other tires and nothing happen to them - they are fine -
Yes mine too. I never mentioned putting a lot of pressure in those.
But on the small tyres theres only around half the side area of the bigger tyres. HALF as much force trying to keep the tyre pushed onto the sides of the rim. So they NEED more pressure. And also because they are so stiff, the tyres sides have less flex, so much more force pushing the tyre around the rim as it deforms under load. This makes them walk around the rim. If they turn maybe 1/4 of a turn during a trip the tube tears. This only occurs initially due to a lubricant being used to fit them. Eventually they will likely "stick" in place.
Its completely irrelivant what pressure you use in the much larger volume 9 or 10 inch caster tyres. You are comparing a totally different tyre and size.
With these you NEED at least 30psi minimum. And preferably a lot more. I have 50psi in mine and they drop to around 40 in a few weeks. So I top them up regularly. You cannot run 20 in those tyres. If you try they will likely tear the tubes. I presume both were damaged?
Yes the ride IS harder. It has to be they are smaller tyres, less sidewall, less volume (by half) and if that causes you problems they are not for you.
I only use them on the Q700R chair, as it has front suspension. Which means they feel fine.