actually I think cavalry has a pretty low mass , but a high charge bonus

don't you remember a guy complaining that cavalry had not enough mass ?

mass value determines which unit "pushes" more , that means if you have 2 units with the same stats except mass , the one with the higher mass is going to "Push" back the other unit

and as far as kill rates , I think its the chance of killing an enemy with every hit...

hope I'm not wrong but I'm pretty sure that's how it works