There has been some discussion regarding how much XP AoE killing groups have gained.  The developers have announced a disabling of the penalty until further tweaks to the system can be made:

As many of you may have noticed we introduced an "Overkill" XP penalty system in version 1.0.1 that was targeted at reducing the amount of bonus experience certain careers (or combination of careers) gained in very rapid, mass AoE killing situations.

Click "More..." below to read the rest.

The change was specifically targeted at bringing the amount of XP gained more in line with the XP that an average player ( or group )could achieve. We firmly believe that AoE capable groups should get a bonus to their experience and level faster.However, thatbonus should not eclipse any other form of experience gain by such a large margin as it had in the past.

Unfortunately, since releasing this system to the public servers we have discovered a small number of instances where the lower limit on the system was affecting solo players too dramatically. As such we are temporarily disabling the penalty until we can properly evaluate and fix these occurrences.We are committed to offering a fair and balanced playing field to everyone.