such an old post, but i'm having the same issue!! lol every time i follow the same steps as you laid out, i STILL cannot get them to be close enough to the same size... and i find this frustrating when i'm trying to make the globey thing... (yes, super technical term.. if you can enlighten me on that, i would be forever grateful! hahaha)
Make sure that if you are making more than two hoops of the same size, that you always measure all of them from the first one you cut. If you use the second one to measure and cut the third, the third to cut the fourth, etc, you will be introducing any errors in measurement into the whole series of hoops.