wangan, are you of 'average' weight for your height? Studies show that men with a heavier build have smaller penises... BUT, when they lose the weight, their penises are actually longer than they were when they were heavier. Look up Dr. Mehmet Oz on Google. He has done some research into this, actually.
Additionally, if you are of average weight, puberty doesn't end when you're 18! Penis size can increase well into your twenties. Your body might not be done growing yet.
The bottom line, is, however, that size really doesn't matter. Generally, only the first 3 or 4 inches inside of a woman's vagina have the sensitive nerves that cause pleasure. Really, it boils down to how well you use it! Be proud of what you have