The experience given by a unit is determined by their level.
The formula is [(Level of (Unit Being Killed)x20)], which means the unit grants 20 experience for each level it has.
Example:
A level 1 unit grants 20 experience when killed;
A level 2 unit grants 40 experience when killed, etc.
These values can most likely be changed in Advanced -> Gameplay Constants.
Remember that creeps also grant less experience as heroes grow stronger, but that's another data in Gameplay Constants.