Greetings,
Some brass are thinner walled. Such as early R-P, Western, and some of the IMI you have come across.
Contact Bruce Williams at Star Machine Works. He may have a smaller diameter Carbide sizing die.
The fancy die was a LifeTyme 2S. It had two Carbide rings. One for the basic sizing and the second sized only the neck area undersize. They work great as I have one that I bring out for thin brass.
It is possible that you might be able to use a smaller diameter expander plug. For the 0.451" jacketed bullets, a plug that measures 0.449 may do the trick.
More measuring is needed.
Cheers,
Dave