I'll do a similar assessment of the Norse segment, since I'm most familiar with it, and because I think it is a good illustration of why giant beings are at least sometimes featured in such myths.
Technically speaking, Ymir did not create the world, he was the world (well, specifically, "the world," minus Muspellheim, Niflheim, and Ginnungagap, which already existed and actually created Ymir, himself; and possibly only Midgard and Asgard, specifically, though presumably the other worlds, too, given the dwarves were also created at the same time). His body (even before Odin, Vili, and Ve killed him) was the source from which the materials to make the world and life came. (Gylfaginning) It's a potentially subtle (on the surface), but rather important difference.
The world is freaking huge. Trees, left to grow for a few hundred years, can grow several feet in diameter and a couple hundred feet tall. The sea, even when out on it, stretches as far as the eye can see, and the sky was untraversible entirely. The planet, itself, is so huge that even modern humans have a hard time wrapping their head around the concept of its size (yes, we can measure it and can cite the distance of its circumference or diameter, but it's still difficult to fully understand what that size actually means).
We create things all the time, so someone had to create the world, right? We create things larger than us (houses, boats, etc), but generally require materials to also be larger than us (namely, trees), even if the materials aren't as large as the final thing, or if we use different parts of the source material for different purposes. So, the gods would have had to have a source from which to create things, since they were the creators. That's the hypothetical idea, anyway (unfortunately, it's impossible to know why ancient people created the stories the way they did, we can only interpret the information we do have, find patterns and whatnot, and form hypotheses).
It's hypothesized by some scholars -- namely Finnur Magnússon* -- that Odin, Vili, Ve (and, for that matter, their parents, Borr and Bestla), as well as Ymir, in the myth are representations of the primal forces of nature. Additionally, many parts of the natural world are, in other nordic myths, created/handled by Jotunar, are Jotunar, or handled by descendants of Jotunar, lending them again to the role of the primal/primeval forces. The stories, then, are a way of understanding forces that otherwise seem completely random, nonsensical, or otherwise mystifying (in Norse lore, this continues on beyond the creation myth, with the sun and moon as beings unto themselves, complete with parentage).
*(Using this link, because it seems and English version of Lexicon Mythologicum is not readily available online. Here's the original-language version for those interested. If someone knows of an English version, I'll be happy to link it.)