Thinking about this more - I believe that introducing a 3rd size watch at such a close sizing to the 44mm, for example, would lead to one issue: Looking back recently at threads for when the S5 was released - I noticed that there were a lot of people who were ordering both sizes 40/44 online and then trying them out at home - then returning the one they didn't want. I remember thinking wow - Apple must be getting a lot of returns of perfectly fine watches that have been opened.
With 3 sizes all so close, I could see that becoming more of an issue for Apple with even more returns.
What does Apple do with those returns where someone just tried it for sizing and returned it?