It's a supply and demand thing I figure. They can probably get discount pricing ordering mass quantities of 12" but have to order one-sies on the odd sizes.
That was a function of pricing I learned when I noticed that dubbed Anime was usually more expensive despite the obvious extra production costs. Because more people were willing to purchase it dubbed they were able to turn a profit on volume.
Then again, it could just be some kind of twisted conspiracy.
Not a scientific comparisson by any means since they were all from different manufacturers but the 4" UV bulbs I have are brighter than the 12" ones. More than likely it's just a difference in companies but someone on the modding forum may have information about actual mcw output of different companies.