"Are you telling me that never once has a case sized with a Lee Collet Die by you or your shooting associates left cases with necks so large bullets weren't held securely?"
I'm telling the world that's so, at least not because of the die itself; that's a case problem, not a die problem. As necks are worked by firing and sizing they get harder and that increases springback until the neck won't stay "sized" in a Lee collet die and the brass soon spits. The well known correction for hard necks is annealing. Anneal after every 4-6 reloads, as we should anyway, and we won't have large/loose necks no matter how they are resized.
"Have you never had to turn down the mandrel to get case necks sized enough?... By enough, I mean bullets were left so loose they could be seated or removed easily with ones fingers.
Sure, I have turned Lee collet mandrels down and also turned a few new ones on my old metal lathe just to try other sizes. What I've learned is that what Lee provides is about as good as it needs to be.
NO metal cylinder (bullet) sitting in a metal hole (neck) that's a thou smaller than it is will spin or be pulled with fingers; that just can't happen! It's possible, but not easy, to do it if they are exactly the same diameter, metallic moving parts virtually always have some bit of clearance or the parts will seize...
I drink no one's Kool-Aid, I am a "fan" of what works well no matter what color box it comes in.