Chrome already does all that for bookmark syncing. The list of +1'ed websites will never grow _very_ large, because it is created one manual click at a time by

Chrome already does all that for bookmark syncing. The list of +1'ed websites will never grow _very_ large, because it is created one manual click at a time by a single user. 10,000 urls is only about a megabyte, and if you send diffs you'd rarely have to download more than a small fraction of them.