We investigate 129Xe-Cs (D1,D2) spin exchange optical pumping (SEOP) at high Xe densities (∼0.12–2.44 amagat) using newly available high-power (>40 W) laser diode arrays and compare with 129Xe-Rb D1 SEOP under similar conditions. At elevated Xe densities, the spin-exchange rate (per alkali-metal atom, γ') for Cs-129Xe is ∼1.5-fold greater than that for Rb-129Xe. Higher spin-exchange rates and lower 129Xe spin-destruction rates for Cs-129Xe versus Rb-129Xe contribute to ∼twofold improvement in 129Xe nuclear spin polarization measured at 9.4 T—with the largest gains observed at the highest Xe densities.