About unit mass, I think all it does is throwing the enemies to the air in the charge and pushing them to hell during melees, is this confirmed? What is the real use of mass, holding/breaking a phalanx seems probable...
Currently messing a bit with this. The mass value effects how soldiers push into enemy formations. I think it effects charging because cavalry have a high mass, therefore push into infantry, therefore the second and third rank soldiers have a chance of attacking with their charge bonus and not just the first rank.

Infantry with a mass higher than 1 seems to be a bad idea so far as they get out of formation without getting the benefit of the cavalry level charge bonus. Testing it to see if giving galatians higher than 1 means they push into phalanxes to simulate the way they were supposed to roll under the pikes of phalanx units.