Going by weight there doesn't really seem to be a reason the Ki-84 should be less durable than the F4F. As far as the P-47 goes, I think it is simply so large that it catches a lot of rounds that would miss a smaller fighter like the Ki-84, and that hides its durability. I certainly feel that about the Mossie, which absorbs AA fire and snap shots like a champ, but crumples when somebody has a bead on it. I think the Bf109 seems tough to me because its size causes a lot of rounds to miss by inches or a few feet that would have hit a larger target like the P-47.