Ok, first impression - this behaviour is as expected! Reason: Object accelerate different due to their mass and friction.
Lets try to solve your problem the other way around. What behaviour do you want? Maybe there is an easy way to archive what you're expecting!
BTW: As I understood from some posting of Ashley - the actual mass of an object is detemined by the set mass and the size of the object...I think he even posted the formula...