Just comparing

I didn't take this photo intentionally for comparison, but they are all but lying parallel to each other with the front of the cranium level, so a good way to show why Cavaliers are classified as brachycephalic on the Cephalic index. Looking at the photo you will see line B indicates the front of the cranium, and line A is the back of the cranium for Toby (Cavalier) on the left of the photo, and for Henry, it is not. He has around another couple cm's at least cranium beyond that line. Line C indicates where Toby's muzzle ends, and again you can see Henry's muzzle extends considerably further. Cavaliers are often not considered short nosed, but that is when they are compared with other breeds with Brachycephaly, that have extreme shortened muzzles like the Pug and French Bulldog. Compared to a dog head with medium proportions though, they are clearly short muzzled. Henry is a Brittany, and is classified on the Cephalic index ​as mesocephalic "having a head of medium proportions, not markedly brachycephalic or dolichocephalic."
​So everything Henry has in his longer cranium and longer muzzle, Toby also has, but it has to  fit in to a lot less room. So we have a concertina effect on the head from the back of the cranium, due to the shortened length of the cranium, and also from the front of the cranium, due to the shortening of the muzzle. Which should be then easy to comprehend and visualize why we see brain prolapsing issues at the back of the skull, and constricted airway issues, as bone, cartilage, soft tissues and teeth all struggle to find space with dogs being bred with Brachycephaly and understand why breeding  Brachycephaly in to dogs intentionally, should stop. 

Toby on the left is a Cavalier King Charles Spaniel, and Henry on the right is a Brittant Spaniel

Most people think the problems with breeds bred with the disease Brachycephaly is all to do with the shortening of the muzzle, and many problems are caused by this, and the short muzzle does further compound the issues caused by the the shortening of the cranium, but as Cavaliers are one of the longer muzzled breeds being bred with Brachycephaly, they are also one of the commonest being treated for BOAS. "In a study of BAOS surgery on 155 Australian dogs, the cavalier was the most common breed." Which indicates that it is not just nose length causing problems and not only should breeders be breeding for longer muzzles to stop dogs suffering with BOAS, but they should also be looking to lengthen the cranium, to not only tackle Syringomyelia, but to reduce the occurrence of  BOAS. So basically they should stop breeding dogs with  Brachycephaly.