I'm wondering if this isn't more of an artistic decision than a scientific/ historic/ mathematical one. Perhaps personal preference is the controlling factor. As seen above, some generally prefer large setts and others, the smaller one.

Each of us have our own ideas about 1) what we think looks good overall and 2) what we think we look good in.

For instance, speaking only for me'self, in the picture at the top I prefer the larger sett for the MacDonald of the Isles, but if I were called on to specify the tartan worn by the gent on the extreme right, I'd like the sett smaller. Why? No idea...just the way I see it.