Hi All,
Craven makes an excellent point about the type of grain making a difference,..but I do think that would have more to do with the thickness than the size. I have a sneaking suspicion that the answer lies in the fact that it's just another example of we americans getting our hands on something, and changing it to suit our needs and tastes, and then being able to mass-produce it at a low cost.
Hers's a link to some interesting tortilla info:
TORTILLA FACTS