I'm 19yo male 300lbs 5'10". My penis is 3.5 in erect. Why is this? What causes it to be small? Anything I can do? Does size matter to you personally?

Lose weight. You can lose weight. Because of your weight your penis seems small, but the fat only makes it seem that o.
Penis size & Obesity. You have two big strikes against you. Being this much overweight means that your natural testosterone levels are low and your natural estrogen levels will be too high which leads to testicular and penis shrinkage. Also being overweight will lead to fat taking up some of your lower abdomen and thus making it appear that your penis is shorter than it would be if this fat layer were thinner.