We study a cooperative problem where agents contribute a certain amount of input in order to obtain a surplus. We assume that the average surplus with respect to the amount contributed is increasing. Within this basic model, a cooperative game is associated and the proportional distribution arises as a natural core allocation. We describe a necessary and sufficient condition for which the core of the game shrinks to the proportional distribution. Furthermore, we characterize axiomatically the proportional distribution by means of three properties: core-selection, core-invariance and resource monotonicity. Finally, we provide a condition that guarantees that the proportional nucleolus coincides with the proportional solution.