Frankly, in high school, I won’t say being fat was a good thing.  But many of the guys on the football team were fat.  I think they saw the guys around them and decided that by pure bulk, they would take down as tacklers.  In addition, you see all the steroid based guys in the major leagues now, and they’re huge, sometimes fat, but they have such agility.  The guys on my high school football team did not actually have this agility factor.  Frankly, there are plenty of teens who have this problem, and not all of them are actually blocking for a football team.  While your metabolism is faster as a teen, a lot of teens are also known for eating junk food and otherwise, and it’s getting worse with younger generations as they grow older.  In addition, exercise is getting smaller.  If you are an athlete, the answer is not to bulk up with fat that will weigh you down, but rather bulk up with muscle.  You may look smaller.  But you will be much more agile and otherwise able to actually block quite effectively without just using mass size, but actual strength.  If you are not an athlete, get in the exercise you need and watch your diet in general.  Instead of going to the pizza line at school, try the salad line now and then for example.