The Fayans energy density functional (EDF), and in particular its pairing sector, have been claimed to be able to reproduce the experimental data of charge radii in many instances. A particularly intriguing case is that of the $ $ isotopes between $ A = 40 $ and $ 48 $, where charge radii exhibit a "bell shape". In our work, we examine the microscopic origin of this behaviour. The pairing part of the Fayans EDF contains three essential parameters: the overall strength $ f $ and the density-dependent and density-gradient terms, $ 0 $ and $ {} $. We select a number of parametrizations, fixing the overall strength $ f $ to reproduce the pairing gap of $ {}^{44} $. We find that some parameter sets enhance the charge radii in open-shell nuclei, and reproduce the bell shape of the charge radii of $ $ isotopes. It is found that this effect originates from the repulsive nature of the rearrangement potential, and cannot be mocked up by a refit of the pairing strength. On the other hand, none of the parametrizations that we have considered can reproduce satisfactorily the experimental charge radii of $ $, $ $, and $ $ isotopes at the same time.