You might be a late bloomer or simply undersized but to compete in the clinches with the big boys you need an adequate amount of mass to absorb contact without going to ground.
Training for size is important but not as important then eating for size. Training wise you’ll probably be training more and harder than usual to gain some added mass but you can only increase mass when you are taking in more energy/food then you are taking out so this means that with extra training comes extra eating.
Do this the easy way by following the Aussie Rules Training WEIGHT GAIN formula that will tell you exactly what and how much to eat to gain mass but also limiting fat gains as much as possible. There is no set food plan, just recommendations to follow.