The way it works is this:
Each unit gets the same base XP.
That XP is then put through a function to reflect the party's situation.
XP = (X)/(M)(.5) For Troops
XP = (X)/(M) For Champions
Where M is the number of champions in the group, X is the XP value from the enemy you killed.
The more champions you add, the less everyone gets. Troops just get what everyone else would have gotten divided by 2.
It is pretty simple but far from explicit.