do the cruise bellows get bigger with age, or are replacement ones just different?.

No and no. Maybe you have the wrong one.

barratts supplied so should be correct

It seems to me that they are manufactured of a different material, which is stiffer; this does seem to limit travel. In practice, doesn’t seem to effect the operation. My replacement bellows is about 7 years old now and doesn’t seem to have softened, which may indicate that it will last (or not).

Stretched to within 3/16" less than original with the help of hot air gun

Related question: In my 1988 XJSC HE, the cruise bellows is mounted underneath its shield. Is this correct? Based on my scanning of various photos of similarly aged V12s, the bellows typically are mounted (~80% of the time) above the shield. This would make sense to me so that the bellows are more protected from the heat of the block. But mine is the opposite, and I've seen enough similar examples to ask: What is the proper set up? Should I reverse it?

No, shield is under bellows to reduce heat stress on it.