This is the result of over leveling. When you are around 4 to 5 level higher than the mob, you practically take no damage and receive lousy exp. If you want to earn exp more efficiently, then you need to know how to harvest mobs a quests. Quest exp wont change no matter what level you turn in, but mob exp will change depends on your level. Mob exp will have a slight drop when you are 2 level above it and will continue to drop even more when you 3 to 4 level above.
