Well, Im not a puppeteer, but I read alot, so I tend to absorb information. This is my understanding. And from what I've read, puppets can get heavy. Someone like Kermit would weigh alot less than someone bigger (Kermit has no foam in his head, unlike someone like Piggy or Gonzo). Also, when you're constantly holding something on the top of your arm all day, your muscles will get strained, and the object will feel heavier. Your arm would have to become stronger, after repeating this routine all the time, so that it could have more endurance.
It's just my putting two and two together.
Maybe I'm wrong, but it seems pretty logical.