Various relations between
sharp isoperimetric inequalities and volumes of manifolds are studied. In
particular, we introduce and estimate sharp isoperimetric constants τ∗ and γ∗
corresponding to two types of isoperimetric inequalities. We show that for a complete
n-dimensional manifold M with Ricci curvature Ric(M) ≥ n− 1, the volume of M is
close to that of Sn if and only if τ∗ is close to n(n − 1)∕2(n + 2)ωn2∕n
and M is simply connected (for n = 2 or 3), or γ∗ is close to 1 (for any
n ≥ 2).