I do not know the formula, but there was a stage when casting Wither on an enemy reduced the amount of xp one got for killing them, so their attack value is probably included in the xp calculation somehow.

Experience is straight up collected from the enemy's Combat Rating, which is a hidden value. You receive a very rough estimate when you look at an enemy and it tells you whether they are Strong, Lethal, Epic and so on. Units that fight for AI factions also give significantly less experience.

But other than that, generally it goes that the stronger and more dangerous enemy, the more experience you'll get. Since there is such a vast amount of variations of each monster (even a single black widow can come in some 400 minor variations) the devs probably thought there was no point assigning units individual experience rewards.

Some traits for monsters do have combat ratings, but I don't know if the value is added flat or something else.

Once the total combat rating is resolved for all the units in the army it is then multiplied by 

<CombatRatingToExpMultiplier>0.075</CombatRatingToExpMultiplier>

which is then split up depending on whether the unit is a champion or trained unit, henchmen fall under odd rules.  After that it just depends on who lives and dies as to whether they receive experience.  That line of code is hidden within the ElementalDefs file.  You can also adjust how much experience total is given to trained units within the same file search for the line,

<ExpMultiplierOnDesignedUnits>0.5</ExpMultiplierOnDesignedUnits>.