We present a systematic study of the stability of the collinear and noncollinear magnetic states of RMn₂Ge₂ (R=Y,Ca) compounds as a function of the lattice parameter so as to simulate alloying with La, using density-functional theory calculations. The results allow us to discriminate between chemical and structural factors that determine the magnetic properties of these systems. We find that, to a large extent, the magnetic moments are determined by the interatomic Mn-Mn distance, given by the size of the substitutional atom. We also find that the different magnetic structures appearing along the phase diagrams are to be mainly ascribed to the interstitial electronic density related to the divalent or trivalent character of the R atom.