Well, some of the weight is in the new rubber mountings, that adds a bit of tonnage. And of course from 2008 on they're all EFI; no more carbs. They are not simple machines any more.
By the way, I was actually referring to my Sportster, not to Sporties in general; the new rubber mounts are still less porky than, say, a Dyna. My '02 is likely going on the block, since over the past year and a half it's accumulated less than 1000 miles, while the Ural has gone everywhere.