A systematic study of the dependence of mean drop diameter, d(32), and protein adsorption, Gamma, on (1) whey protein concentration and (2) hydrodynamic conditions during emulsification is performed with soybean-oil-in-water emulsions. We find experimentally and explain theoretically an interesting interplay between Gamma and d(32) in the studied systems: At low protein concentrations, the mean drop size is governed by a critical value of the protein adsorption, Gamma*, which is a characteristic of the emulsifier and does not depend on the hydrodynamic conditions. On the other hand, at higher protein concentrations, Gamma is determined by the initial protein concentration in the aqueous phase, C-PR(INI), and by the mean drop size, d(32), the latter being governed only by the hydrodynamic conditions. The theoretical model, developed to describe these relations, is verified by comparing its predictions with experimental results obtained at various protein concentrations, oil volume fractions, and hydrodynamic conditions. The model allows one to predict the dependence of various, technologically important quantities on the emulsification conditions.