You also have to remember that the pixel density of a smaller screen is alot higher than a standard 15in screen.
I guess its mainly to do with demands. Why would a VGA monitor cost alot more than a composite video monitor? Same screen, same size. I dont think the VGA controller is so much more complicated than the composite controller.
Ill say the small video monitor is more common and much more in demand and therfore more competitions, lower price, more sales.
With the bigger monitor, again thats low in demand but also they are harder to make as you need a bigger piece of silicon in one piece.
Try and buy a big piece of wood in one piece and compare the price of the smaller piece joint together to make a big one. Not easy to get a tree that big, thats the reason for the high price...off cuts are cheap.