I noticed yesterday how easy it was to kill even packs of 4 mobs in cyrodiil while collecting skyshards and books on a new level 16 char
It must have something to do with battle leveling. I think they gave me 95xp each.
Get an addon like Advanced UI or Foundry Tactical Combat and you can tell exactly what each mob gives you for Exp.
Cyrodiil Mobs are really VR5, but are now marked as VR12. Yes, higher ranked mobs give more exp, but not so much more that I would go out of my way to grind them. Then again, I am not a min maxer who has to optimize everything I do in the game. i just like to have fun.
However, questing is still the best source of Exp and leveling, and you get plenty of mobs to grind on while doing the quests.
Believe it or not Questing seems to be the best way to level up. Cyrodil is more for farming their quests are too far from each other and their mobs don't give much exp/gold because anybody out of group can help you take down these creatures so its kind of a fail safe. Try doing a bunch of quests within range of each other and see what happens. If all else fails just keep doing regular dungeons or Pledges. Veteran dungeons I didn't see a difference in exp