There may be a correctable cause, a Doctor would be able to check. It may simply be how you are built. It is the same as asking why you have the hair and eye color you do, why you are the height you are or any other physical trait. Everyone wants 10 inches and it just doesn't happen. How big your penis is does not define who you are or what kind of a life you will have. It is small, the average for a 15 year old is four inches.
think about Angelina jolie or Eva mendes naked.
4x regular size
The stock barrel is 11 inches. The receiver is nearly 12 and 1/2 inches long (including butt-cap). the butt-stock is also about 11 inches fully extended. That makes the gun 34-38 inches long, depending on stock lenght.
The are not fully extinct but nearly extinct.
They are about 1-2 inches when fully grown.
Fully mature, about 4 inches.
30 inches diameter or 93 inches circumference
it is 13-16 inches high
five inches to a foot long
8.5 metres
A gibbon can be 16 and a half to 23 inches long.
about 2.5-3 inches depending on the sex