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 similar to M-KK/M-P similar to V-2/3 << 1 and argue that consistency of the superspace derivative expansion requires gF/M-2 similar to 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.