Exactly - the hole in the middle of the ring is too big for tiny breeds and if done too early would cause them a lot of pain with the blood supply to the testicle being only partially cut off. With the legal limit set at 7 days, I think those who can't do it within that time because of the small size of the testicles will not want to raise the problem in case they are penalised We don't ring many of our Hebridean males because we like to see how they turn out to have a choice to pick the best from for selling on as breeding stock, but sometimes it's obvious one isn't suitable so he will be ringed, and with Soays we do any we don't already have a booking for as they can be confrontational little devils once they get in with the boys flock, setting off fights which the Hebs never would, so wethers are easier to manage.
Going back to the original question, lambs shouldn't squeal when they are ringed or make more fuss than going to sit down for a few minutes, or doing the funny walk briefly. After that it shouldn't cause them any problems, so if they were in a lot of pain I would assume we had done it wrong.