I've never quite figured out the giant blades, myself. Sometimes they are just too big to be explained by an extra-large owner. Here is a super-sized dha, which even leaving aside the blade had to have been made for a HAY_UUGE person if it was made-to-fit:

Just compare the handles of these two (the blade of the top one is of a pretty typical length, actually (about 22 inches, or 58 cm), and the lower one's blade is on the short side at about 16.5 inches, or about 42 cm). But the handle of the top one is unusually long for this style of dha, and has a diameter about half again as big as the lower one, which is a more typical length. I am 6'1" (1.85 meters) and the gripping part of my hand, the distance from the end of my thumb along to the end of my fore-finger, is around 7.5 inches long (around 17 cm). The lower handle feels fine, but the top one feels funny because it is so fat, and ones grip on it feels insecure and uncomfortable.
I just don't think that there were many Thai around in the 18th-19th century who were significantly taller than I am. My guess is that it was either a status symbol of some kind, or ... um ... overcompensating for something.
Here is another fun one:

This is a Vietnamese mandarin and his entourage, circa 1900. I am guessing that the big guy on the far left may be around my height (and he doesn't look to be ethnically Vietnamese, so he doesn't count as a guy they might have made huge swords for

). Now check out in the group on the right, the third and fifth men from the right in the front row. Both are holding dha that I would estimate are about 4 feet long, or more. The sword #3 is holding looks quite a bit longer (5 feet?). These would all be symbols of the mandarin's power and authority, I expect.