Actually, it was recently stated by a developer on #bitcoin-dev that it is an issue of the total set of unspent transaction outputs (i.e., what all nodes must m

Actually, it was recently stated by a developer on #bitcoin-dev that it is an issue of the total set of unspent transaction outputs (i.e., what all nodes must maintain in memory), not total blockchain size (which only nodes that need to do complete validation actually need). Of course, it may be that all the developers are not of one mind on this.