Which function "grows" faster, a linear function or an exponential function and why?
$y_1 = mx + b \Longrightarrow y_1' = m$ where $m$ is a constant
$y_2 = a^x \Longrightarrow y_2' = a^x ln(a)$
It should be clear that $y_1$ is changing at a constant pace whereas $y_2$ is changing with respect to a variable. Eventually it will surpass the rate of the linear function, no matter how big, and then the linear function itself. (But only for certain $a$, and you should verify this).
For each unit of increase in its argument, a linear (increasing) function goes up by the same amount. For an (increasining) exponential function, this increase is by the same multiplicative factor.
So, what do you think now?