If a graph submanifold (x, f (x)) of a Riemannian warped product space (M m × e ψ N n , g = g + e 2ψ h) is immersed with parallel mean curvature H, then we obtain a Heinz type estimation of the mean curvature. Namely, on each compact domain D) holds, where A ψ (∂D) and V ψ (D) are the ψ-weighted area and volume, respectively. In particular, H = 0 if (M, g) has zero weighted Cheeger constant, a concept recently introduced by D. Impera et al. ([13]). This generalizes the known cases n = 1 or ψ = 0. We also conclude minimality using a closed calibration, assuming (M, g * ) is complete where g * = g + e 2ψ f * h, and for some constants α ≥ δ ≥ 0, C 1 > 0 and β ∈ [0, 1), ∇ * ψ 2 g * ≤ δ, Ricci ψ,g * ≥ α, and det g (g * ) ≤ C 1 r 2β holds when r → +∞, where r(x) is the distance function on (M, g * ) from some fixed point. Both results rely on expressing the squared norm of the mean curvature as a weighted divergence of a suitable vector field.