Some observations of repeating earthquakes show an unusual, non-self-similar scaling between seismic moment and corner frequency, a source property related to rupture size. These observations have been mostly reported in regions at the transition from stable to unstable slip, in geothermal reservoirs and subduction zones. What controls the non self-similarity of these ruptures and how this is linked to the frictional stability of the interface are still open questions. Here we develop seismic cycle simulations of a single unstable slipping patch to investigate the mechanisms underlying this behavior. We show that temporal changes of normal stress on a fault can produce ruptures that exhibit the observed anomalous scaling. Our results highlight the role of fault zone fluid pressure in modulating the effective normal stress and contributing to the sliding stability of the fault.