Having mic'ed and fitted a ton of YM match carriers, I can say with some certainty that they are all exactly the same size.  I have seen plenty of receivers that had out of spec bores however.  Some that were bored undersized, others that were misaligned.  Many forged upper receivers have a tight spot around the rear of the ejection port.  I mentioned it to Dan Yound several years ago and he did a fair amount of research on the problem.  Bought a selection of receivers from every major supplier.  The variations in dimensions from one receiver to the next, brand to brand, within a brand, you name it, was astounding.  The bottom line was that the clearance he was assuming, based on the minimum bore size in the prints, just wasn't there.  

While YM match carriers are only 10/1000's larger than a nominal carrier, they were tight in underbored receivers.  Because he could not control the folks machining receivers, he changed his setup and his NM carriers are now 5/1000's smaller than they were, only 5/1000's larger than standard.  We occasionally still see a tight one, but not often.  It isn't the carrier though, the receivers are just a little tight sometimes.

BTW, Dan no longer makes AR bolts. Though his bolts were truly works of art, we are in an era of cheap and he couldn't sell enough of them to make it worth setting up a machine to make them. He now buys bolts and has them chromed when he needs to supply complete hardchrome BCG's.

McNamara's team was considered a failure when it came to the adoption of the M16 in Vietnam. After seeing the original design proven in tests, they changed specs to cut costs and sent an unreliable platform into the war. McNamara and his cohorts had many successes, but they made a mess of this one.

Chrome carriers are certainly not a necessity, but they are nice from a maintenance standpoint.