I'm not familiar enough with Japanese history, but in western blades, form followed function.
At first a wide blade was needed in the "bronze age" as you needed the heft to make a blade, because of no iron, blades where wide and short. As we enter the "iron age" weapons became longer because the materials where better. As combat shifted from foot to chariot, blade lengths increased. As armor got better, swords got heavier, those boys with there big 'o swords where wacking at each other like they had baseball bats not edged weapons. After gunpowder and firearms, armor became useless, so no longer is there a need for big bang'em up swords the swords got lighter. Also since your not hitting armor but going against flesh a curved blade slices throw flesh better than a straight one. (whens the last time a hunter skinned a deer with a straight edged knife) Rapiers and the like came about for a particular style of fighting. Cutlass's existed aboard ships cause you did not wear armor on a ship (if you fell off you sank) Here a curved blade was more effecient. And as pointed out by our knowledgeable posters it's easier to draw a curved blade and you get more edge on a curved blade.
Ok, it's a bit rambling, but you see many factors affected the development of differing western style swords. Materials, construction techniques, purpose (what it was used for) etc. etc. So it makes sense to me that differing forces where at work in the Eastern world as well, but form follows function, they look the way they do cause it works!
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~ JerryO ~
Cogito Cogito Ergo Cogito Sum