  Let G be a compactly generated locally compact group and let $U$ be a compact
generating set. We prove that if G has polynomial growth, then (U^n) is a
Folner sequence: that is, the volume of the boundary of U^n divided by U^n goes
to zero. Moreover, we give a polynomial estimate of this ratio. Our proof is
based on doubling property. As a matter of fact, the result remains true in a
wide class of doubling metric measured spaces including manifolds and graphs.
As an application, we obtain a balls averages L^p-pointwise ergodic theorem for
probability G-spaces, with G of polynomial growth and for all p greater than 1.
