Can a average size 15 year old get a big muscular chest?
Most definitely YES! When you are 15 years old you are in the middle of puberty and awash with anabolic hormones like testosterone and growth hormone. You will respond very well to weight training and any other form of physical exercise. I would recommend a balanced training program including compound lifts which exercise multiple muscles at the same time such as squats, deadlifts, barbell rows, pull ups , overhead presses, bench presses. You can work out 3 or 4 days per week, taking at least 48 hours between each session. Make sure you eat enough calories. If you are not overweight you should eat a caloric excess. When you eat about 25–30 grams of protein it stimulates muscle protein synthesis in an all or nothing hormonal response, so make sure to eat plenty of protein. Working out is only part of the equation. As part of the stress-recovery-adaptation cycle your muscles will develop micro-tears after training, and they will grow when you rest and consume enough protein and calories. So train hard and pay attention to rest and diet, and you will soon see improvements.