It doubles because you are exponentiating two, not as a general rule.You see that every time you add 1 to the exponent, the outcome doubles (exponentiation works like that).
3^1 = 3
3^2 = 9
3^3 = 27
3^4 = 81
This tendency for values to rise rather quickly is why very rapid growth is called "exponential" in common use of the language.