If memory serves, it's primarily based upon the number of kills for the whole unit, averaged out by number of kills per soldier.

There are odd times when a unit will gain experience for just being there, which I don't fully understand, unless total kills for the battle are averaged out and added to the (unit/individual) score as well.

The threshold is, if memory serves, based on a number sequence that I can't remember the name of but goes something like this: 0,1,1,2,3,5,8,13,21,etc.

Hope that helps (if I've got it right, of course).