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?
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)
Note that nothing else effects experience, including additional party members.
The amount of experience scales both based on the level difference between your character and the monsters, and also the levels of the other players in your party (if applicable). While I don't have a link to any specific figures on-hand, it sounds like the person you spoke to was referencing the fact that being power-leveled by higher-level friends by grinding on monsters will not give you large experience gains, due to the level difference involved.
There are alternative ways that people have used to level lower-level characters quickly, but I believe those all center more around quest completion XP than XP gained from killing monsters.