Pressureless balls have a thicker (not longer) felt, as they are designed to last more than 9 games. It takes a while to wear it down but they eventually end up closer to the feel of a fresh pressure ball. Would not surprise me if the rubber become less hard as well so the rubber bonds break down, they aren't as rigid.
They are not really designed for normal play, mainly for ball machines which destroy pressure balls and high altitudes where pressure balls have issues due to the pressure differential, pressure balls tend to fly at high altitudes (low atmospheric pressures), theoretically you should use lower pressure balls at high altitudes of thicker felt to increase mass. Not sure if this happens, I never seen them if they exist.