To build muscle mass you should be doing high weight for sets of 2-4 reps and occasonaly 6. When you are lifting lower weights for sets of 8-10 reps you are toning and burning fat and not building as much muscle. The ideal situation would be to do a set of lower weight of 10 reps and then work your way up to doing sets of 2-4 reps with a high weight then as a cool down do another set of 10 reps. Similar to what Kyle Harrison does with his bench press,
YouTube - Kyle Harrison Lacrosse work-out.