Here's the SOE forums post by a Game Designer.
Excerpted for easy digestion:
Summoner Balance
Currently, Conjurers and Necromancers are a very powerful class that can deliver the most consistent damage in the shortest amount of time and can solo more powerful encounters than most other classes. One of the problems is that their single target damage can exceed that of Wizards in solo, group, and raid situations, even when hostile area effect damage is considered that can kill swarm and dumbfire pets.
The next test update decreases the effectiveness of certain pet abilities and buffs that were overpowered. These changes don't necessarily mean that Wizards are always supposed to outdamage Summoners. Summoners can have favorable situations in long fights without large area effects that can tip the scales in their favor above Wizards. It shouldn't be the case anymore that Summoners outdamage Wizards the majority of the time after these changes, and Summoners will still retain relatively powerful soloability even after these changes.