My frame is on the small side for several reasons. (I chose this and built the bike myself without Thorn input.)
I like bars lower than the saddle and a shortish reach to drops. I had other bikes to measure this from which helps.
I don't like "avoidable" weight.
I have no aesthetic requirement for horizontal stems or specific amounts of seat pin exposure. In fact I like long seat pins (MTB influence I think) and upward sloping stems (shortest route between two points, less spacers and shorter steerer, greater rigidity etc- horizontal stem means longer steerer, more spacers, more weight.) Some would say only a man without aesthetic sensibility could like a Thorn, but I side with Andre in the Bauhaus of bikes counter argument ie function determines form and the form is therefore functionally beautiful.
To get my size I started with top tube length/handlebar reach required and used the smallest frame possible (and longish stem) to give this. Standover then sorts itself out. Long seat pin sorts the saddle height and Andy's choice of a relaxed seat angle means no layback post required.
If you are going to have a compact (ie sloping top tube) frame it may as well be a compact (small) frame
Other things being equal it's also lighter and stiffer for no compromise in reliability.
Until I saw Dan's bike with all the steerer mounted accessories I couldn't see a reason for going big
Ian