The resistive pressure gradient driven instability model is used to study the effect of plasma on a vacuum magnetic field island. Conditions under which the magnetic island is amplified or reduced have been identified. Starting with a set of reference parameters, we have varied these parameters by increasing either the density or the electron temperature. These scans lead to very different results. When β increases because of the increase in density, so does the island width. However, in cases in which β is increased by increasing the electron temperature, we observe a decrease in the island width. The main mechanism for island reduction seems to be the generation of strong sheared flow associated with the magnetic island. These results seem to reflect some observations in the Large Helical Device.