Total utilitarianism is a philosophical system of ethics where the good is summed across the people. In total utilitarianism, number of people should be increased at expense of quality of life, up to a point where their product is maximal. (Not all the way to lives barely worth living, though, but indeed to the point of some significant overcrowding where you could have a smaller population living better lives).
Average utilitarianism: an attempt to fix the above, done by mathematically impaired philosophers. Instead of a sum, an average is used. Which only makes it worse.
Suppose there's 10 people being tortured at utility of -1000 each , who you can't affect, and you can choose between 9 additional people living with utility +1 each, or 10 additional people living with utility -1 each. The utilities are such: -1000 is really really bad non ending torture, -1 is suffering enough to make one commit suicide if there's no hope of improvement, and +1 is normal happy life.
So under average utilitarianism you prefer for 10 extra people to live in suffering, than for 9 extra people to live in happiness, just because there exist 10 other people who suffer a lot worse.