Ultrathin InAs nanowires (NWs) can enable true one-dimensional electronics. We report a growth phenomenon where a bimodal size distribution (∼ α nm and ∼ 5 nm in diameter) of InAs NWs can be achieved from gold (Au) nanoparticles of a single size, α (α = 50-250 nm). We determine that ultrathin InAs NW growth is seeded by ultra-small Au nanoparticles shed from the large Au seeds upon indium (In) introduction into the growth system and formed prior to the supersaturation of In in Au. The Au spreading phenomenon is explained by the balancing of Gibbs free energy lowering from In-Au mixing and the surface tension increase. Ultrathin InAs NWs formed in this way exhibit a perfect wurtzite structure with no stacking faults. We have observed InAs NWs with diameters down to ∼ 2 nm using our growth method. Passivating the ultrathin InAs NWs with an AlAs shell, subsequently oxidized in air, results in physical deformation of the InAs core, demonstrating the mechanical pliability of these ultrathin NWs.