I have been breeding pedigree and also commercial livestock for 30 odd years.....I have been involved with cattle, pigs, goats and sheep in the way of large livestock.
There are many dilemmas here ......do we want to conserve every member of a rare breed regardless just because they are rare, do we want to ruthlessly cull to accentuate certain points, do we cull or refuse registration for over height, odd spots etc etc?....
Well as far as I am concerned we don't want all breeds to become homogenised into a single 'type' but there are important faults and less important faults and what I personally perceive as daft faults!
For example good breeding animals need 4 good legs and good feet to stand on, a decent coat/fleece/hair etc to keep out the weather, wide pelvis for easy lambing/calving etc, good heart room to be able to breath and eat efficiently, If horned good horns with which to ...if they were wild....fight with.....etc etc
These are basics and in the past would have been sorted by natural selection...ie the lions or wolves would have got it!
After that the different breed charecteristics were fixed by man a lot of the time for specific properties...even the primitives....as they are adapted for the places they came from...for example the North Ronaldsay is a small sheep that can live on next to nothing and even eats seaweed....which a Texel would starve rather than eat
After that it starts getting pernickity.....imho...!
For example 20 years ago i bred a fabulous Greyface dartmoor ram.....really good! I was very pleased with him! However...he had a few black tiny spots on his lower legs so was rejected as a breeding ram......wheras another that was submitted that was not a patch on him....passed!.....needless to say we ate the one that passed and used the other one on our commercials....with a very good if not conventional result!
I bring this up as you can sometimes literally throw the 'baby out with the bathwater' especially with rare breeds and limited genepools.....
I now keep Ouessants....much to the amusement of my farming friends who think I have taken leave of my senses and call them handbag sheep
We have a limited genepool in Britain and I cannot find a good black ram....I can find lots of black rams...but not a good one (poor horns, legs, feet, narrow body etc)....demonstrating the practice of keeping an animal that should be eaten/castrated...just because its rare/unusual! I however have a few specimens who are a bit overheight....but in every other way are perfect and I know the reason is good nutrition...which as has already been pointed out can cause primitives to grow bigger.....I am not going to cull these I just use a small ram....result is correct height offspring....
Breeds evolve......just like people...who are taller now than they used to be!