Hi There are no medicines (pills, lotions, creams, sprays etc) or exercises to increase the penis size. It is genetically determined and it has no muscles inside it. That's the reason.
EVery penis is beautiful in its own way. And every penis is equally capable. A larger penis may be aesthetically appealing to the eye but functionally no better than its smaller counterpart. SO, the best thing would be to focus on earning sex techniques that give you and your partner a great sexual pleasure and satisfaction, notwithstanding the penis size. How? Read Kama Sutra by Vatsyayana - an all time great sexologist-sage.