The charge and size of a ligand atom and/or metal ion can effect the electrostatic attraction between the two, in turn effecting the attractive force between metal and ligand. This comes from Coulomb's Law. In general, the higher the charge, the stronger the attraction between metal and ligand. And, in general, the smaller the ion, the stronger the attraction (because small ions can get closer to their partner).
For example, with the same ligand set, we expect increasing stability as size of the metal ion decreases, or as charge of the metal ion increases:
Modified or created by Kathryn Haas (khaaslab.com)