I never heard that if you masturbate you will be thin or decrease you size. I'm 21 yrs and I noticed that keeps increasing size and you build up more forearms and biseps.

Yes, like any muscle in your body the vascular and kegal muscles of your penis will develop through regular exercise (AKA masturbation). Over the course of your life daily masturbation with care not to injure your penis might increase your penis length by as much as 10% and likely gain even more in circumference. However, excessive masturbation carries the danger of sexual dysfunction.
Natural body building has no impact on penile health, however the cardio-vascular excercise that is usually joined with bodybuilding will increase both erection fullness and sexual stamina. So good stuff there. Anabolic Steroids and other muscle growth drugs and hormones can stunt the growth of the penis or reduce erection fullness. Be informed of any bulking aid you use if you want a bigger tool. Weight loss or specifically fat loss has a dramatic impact on penis size, unfortunately it sounds like you don't have a lot of pounds you can shed.

no, the only way that i know of to decrease the size of the penis (besides sharp objects)is to take steroids/tesosterone, this will make ur body stop producing its own tesosterone and then when u stop taking them your penis will shrink. the other way is to gain weight... this will add a fat pouch above the penis and hide some of it.