I know that people often get smaller as they get older. But that usually happens in their seventies or eighties. Old Jonathan is still supposed to be relatively young. But it shows how incrementally the idea of Pa and Ma was changed from when Byrne first introduced them in MAN OF STEEL. He probably did mean them to be in their sixties when Clark is full growed--and these gnomish looking farmer stereotypes. But the idea of them kept changing and making Pa a tall strapping youth was one of those incremental changes. It would be another rainy day project to pull out all the old comics and look at how artists kept changing the Kents and making them younger and more Hollywood attractive.