The hygroscopic growth factors of NaCl particles having dry mobility diameters of 6 to 60 nm were measured using a tandem nano-Differential Mobility Analyzer. The growth factors steadily decreased within detection limit for dry sizes below 40 nm. The decrease is quantitatively predicted by a model that includes the Kelvin effect and a size-dependent shape factor. This factor is not tuned to the data but rather is grounded in theoretical predictions from literature. Agreement in growth factors for particles generated by two independent methods (namely, vaporization-condensation and electrospray), as well as observations of prompt deliquescence, indicates the absence of significant chemical impurities.