They just should make smaller groups and single players capable of doing something useful and actually contribute to the war effort. If you need a large group to take down an upgraded basic structure then there is not a smallest reason to ever split from a blob (and btw, I have always wondered who came up with this brilliant notion that internet players love to group up, form parties and socialize (the dreaded “hi, from?”) just to blow their noses – and judging from the amount of dungeons parties’ kicks and whining there might some mistake in this assumption: many people group up only when they are forced to do so by game’s mechanics).
Reducing NPC strength (in PvP mode..), eliminating possibility of camps and towers upgrades, lowering supplies requirements for building siege weapons, and – maybe – creating opportunity to infiltrate structures and temporarily eliminating lords for some quick war points would be a good start for both reducing blobs and increasing wvw populations imo.