The mobility of p-type nanowires (NWs) with diameters of D = 12 nm down to D = 3 nm in [100], [110], and [111] transport orientations is calculated. An atomistic tight-binding model is used to calculate the NW electronic structure. Linearized Boltzmann transport theory is applied, including phonon and surface roughness scattering (SRS) mechanisms, for the mobility calculation. We find that large mobility enhancements (of the order of 4×) can be achieved as the diameter of the [110] and even more that of the [111] NWs scales down to D = 3 nm. This enhancement originates from the increase in the dispersion curvatures and consequently the hole velocities as the diameter is scaled. This benefit overcompensates the mobility reduction caused by SRS as the diameter reduces. The mobility of the [100] NWs, on the other hand, is the lowest compared to the other two NW orientations and, additionally, suffers as the diameter scales. The bandstructure engineering techniques we describe are a generic feature of anisotropic bulk bands and can be also applied to 2D thin body layers as well as other channel materials.