We report first-principles study of the magnetic structure of Mn₃Si. The Mn atoms form two sublattices. One of them is (Mn_I) characterized by large atomic magnetic moments and the other one (Mn_II) possesses induced moments. It is shown that in agreement with experiment the magnetic ground state is a spin spiral. We found that the spiral magnetic structure is formed as a result of the competition between direct antiferromagnetic interaction of the Mn_I magnetic moments and indirect ferromagnetic interaction of the same moments through the magnetic moment of the Mn_II sublattice. We demonstrate that this competition is strongly volume dependent and leads to volume dependence of the wave vector of the spin spiral. These properties are related to the volume dependence of the induced moments of Mn_II atoms