The post is for just the boots, not the entire shaft - and a larger shaft wouldn't fit either. If the boot is made for a larger shaft (H3) then it won't properly fit on the smaller shafts on our trucks. If you don't have a good seal around the CV joint it will get contaminated and dramatically decrease the longevity of the joint.
I didn't get a chance to look at the link while I was on my phone. A larger shaft would fit, if you are talking the mid-shaft being thicker (the part between the two joints) but keep the splined portions the right size. I do agree with you about poorly sealed boots.