Pendant-drop tensiometry of aqueous-buffer solutions of purified human proteins spanning nearly 3 orders of magnitude in molecular weight (MW) reveals that reduction in liquid-vapor (LV) interfacial tension gamma(lv) followed a systematic progression in MW with the molar concentration required to reach a specified gamma(lv) value decreasing with increasing MW in a manner reminiscent of the Traube rule for linear hydrocarbon surfactants. Furthermore, the concentration dependence of interfacial tension (dgamma(lv)/d In C-B, where C-B is bulk-solution concentration) is observed to be surprisingly invariant among this disparate group of proteins (i.e., approximately constant apparent Gibbs' surface excess Gamma = -1/RT dgamma(lv)/d ln C-B). These findings are interpreted through a model of protein adsorption predicated on the interfacial packing of spherical molecules with dimensions scaling as a function of MW. The Traube-rule-like ordering is rationalized as a natural outcome of an invariant partition coefficient that entrains a fixed fraction of bulk-solution molecules within a LV interphase which thickens with increasing protein size (MW). Thus, protein adsorption follows a homology in molecular size rather than composition. Calibration of the sphere-packing model to previously reported neutron reflectometry of albumin adsorption permitted interpretation of tensiometric results in terms of interphase thickness and multilayering, predicting that relatively small proteins with MW < 125 kDa (e.g., albumin) fill a single layer whereas larger proteins with MW similar to 1000 kDa (e.g., IgM) require up to five molecular layers to satisfy a constant partition coefficient.