Two years ago we played up a grade U13's playing U 14's and every game was against bigger opposition.   This was fairly daunting for the boys.    There are not many ways to counter the size factor but one that worked for us was low tackle drills.   Shoulder hits thighs, head against side of leg, hands wrap around the back of the thighs and slide to behind the knees and then pull the knees forward at the joint.   The  player must fall forward and over your shoulder and you have to keep hanging on in the tackle until the player hits the ground.