My hands aren't that big I don't think, but I don't get picking DSLR's based on the grip. True, the XT's are small and my pinky is forced to hang off. And I have to jam my ring finger to get it on there. It can't fully support the camera. However, the same is true of my 30D, and in fact, I went to Fry's, which actually puts 5D's on the floor, and the same thing. It feels nicer, but I still can't support it with all five fingers.
I can tell the 30D from the XT more by texture and contour than size. The 5D's thickness is more noticable and the indent for the index finger is useful, but it doesn't significantly alter how I have to hold it. Or my need to use my other hand to shoot 99 percent of the time.
A grip may help, but for me, I am guessing a grip would help all about equally. At least not qualitatively differently.
I had reasons for getting the 30D and I have reasons for lusting after the 5D, for many of the same reasons most others here do. But, y'all lose me here.