A note on the magnitude of the flux superpotential

The magnitude of the flux superpotential W flux plays a crucial role in determining the scales of IIB string compactifications after moduli stabilisation. It has been argued that values of W flux ≪ 1 are preferred, and even required for physical and consistency reasons. This note revisits these arguments. We establish that the couplings of heavy Kaluza-Klein modes to light states scale with the internal volume as g ~ M_{KK} /M_{P} ~ V^{−2/3} ≪ 1 and argue that consistency of the superspace derivative expansion requires gF/M^2 ~ m_{3/2} /M_{KK} ≪ 1, where F is the auxiliary field of the light fields and M the ultraviolet cutoff. This gives only a mild constraint on the flux superpotential, W flux ≪ V^{1/3}, which can be easily satisfied for O (1) values of W flux. This regime is also statistically favoured and makes the Bousso-Polchinski mechanism for the vacuum energy hierarchically more efficient.