What determines how much experience you get from a kill?

Someone told me that monsters don't give a static amount of experience; if that's true, what determines how much experience you get from a kill? What's the exact equation?

Solution 1:

There are two factors that change how much experience per kill is given. These factors modify the base experience of the monster, each monster has a different base experience.

First is the monster level vs your level. This chart is from Malloy626 on Reddit (click for larger image) demonstrates the approximate experience gain from monsters in each act and difficulty relative to your level. Malloy626 summarizes that the optimal leveling path is the yellow zone in the chart.

This is based on the following experience chart (monster level vs character level)

char level +10 or more of monster level = 0%
+9 = 1% (+exp effects nullified)
+8 = 5% (+exp effects nullified)
+7 = 15% (+exp effects nullified)
+6 = 30% (+exp effects nullified)
+5 = 45%
+4 = 60%
+3 = 70%
+2 = 80%
+1 = 90%
+0 = 100%
-1 = 115%
-2 = 120%
-3 or less = 125%

Second is gear with +experience (example) or +%experience (example) per kill. This simply multiplies the amount of experience you get after the level disparity modifier is applied.

Note that nothing else effects experience, including additional party members.