Magnums are supposed to be 60mm where XL's are 65mm. I've never used an XL for sex, but i've tried one on just to see if it fit and it didn't really feel much bigger than a regular Magnum.
I've used Magnums a few times... I still have some but don't really like them. They feel kinda thick. I prefer Trojan - For Her Pleasure. They are dual-sized in that they are very snug (regular size - 52mm) at the bottom half and bigger (XL size - 65mm) at the top half. They kinda act like a
cockring at the base and about as snug as XL's toward the tip.