If you are going to create large open world environments that you want to strictly PVP in, you will have to artificially create separations and they have pros and cons. Here are some retarded ways to do it:
1) Artificially limit population whether by server, instances etc.
2) Artificially punish players for blobbing up. (Debuffs with -damage or whatever.)
3) Create so large a map with long travel times so people stay in local areas. The con here is that people say fuck it, I'm not walking back and log off.
There are more subtle ways to do like interweaving PVE and PVP together and make sure you don't care if player's PVE is either shutdown or harassed. In order to get smaller groups, or spread people out, you need to make the gameplay not solely about PVP. What I mean by this is that your pvp game needs to have farming, leveling up, searching, exploring etc. In GW2 and what I assume is in ESO, you have a game map where all you do is kill people or kill castles. When you zone into the map you're there for killing shit. You're not there for doing other things. Those other things should be gear farming, crafting, leveling or whatever.
You need to force the players in to making play time decisions. Do I sit here and zerg and get saturated kill totals? Or do I spend my time more wisely hunting down people solo or in small groups? Or do I spend my time farming a better weapon (with a chance to do small scale PVP on the way)?
Essentially your whole game needs to be in that map. The least efficient method should be zerging and getting experience and badgets/loot from the graveyard of the other zerg. Honestly you should not be rewarded at all for zerging.