< if the breed is evolving then is it not better to let it evolve? >>
Well, there's evolution and evolution
One of the biggest influences on domestic animals, as I mentioned above, is the effect of
human selection, deliberate or unwitting. We all have a tendency to choose the biggest animal, particularly when judging in a show - the biggest will probably be the most impressive. These animals, or their breeders output, will tend to be favoured by other flock owners who will then buy their stock, and this will, and does, lead over the years to a gradual size increase in the flock. Other things can be selected for by the same mechanism - fleece type and colour for example. This may be deliberate, such as in the case of the Manx Loaghtan where fleece suitable for spinning and making hosiery was chosen, or a sort of by-product of showing, as in the case of the Hebridean, whose fleece has become longer and blacker in the years since the HSS took over management. Many breeders are quite happy with this because the really black Hebs are very striking, but others feel that the breed is losing a primitive characteristic which helped the animals to survive in their natural home of the rocky margins of Britain.
Another characteristic which may be unwittingly selected against is ease of lambing, and lambing successfully in challenging conditions. Again with Hebs, they are now mostly bred on mainland Britain, and many breeders lamb them indoors, with full support - which is of course perfectly natural - you want your animals to survive. But this is preventing the other type of evolution - by natural selection - which is governed by survival of the fittest, in this case the fittest according to their original home. So the animals may well now be doing fine in the current environment, but if we want to preserve the original characteristics of an animal totally suited to a particular local environment, then we should be careful to preserve those characteristics, at least in a nucleus of the original type. These animals and their genetic traits will then be available for us to use when needed in the future - and of course the future starts today.
I don't think that a wide variability of characteristics is such a bad thing, as these are all part of the rare breeds, as opposed to more recently created breeds where a strict standard is applied for pedigree stock. For all those who like bigger specimens, there are likely to be an equal number of breeders who keep the opposite type, but it is the breed society's duty to keep an overview of what is happening, to make sure this balance is maintained.
It's not a case of policing, and what you can get away with. If you keep a rare breed then surely you do so because you want to preserve that breed. You are a
member of a society set up for the preservation of that breed - it shouldn't be a dictatorship and if you feel strongly about something then you can always get yourself elected to the committee.
The usual way to get a bigger carcase for meat from these smaller breeds is to cross them with a larger breed to give cross-bred lambs.