The present study on a tensile-deformed superplastic alumina reveals that the growth of cavities finer than the current grain size (fine cavities) into larger ones (large cavities) is constrained from the surrounding matrix. As a result of the constraint, the density of the large cavities becomes noticeably lower than that of the finer ones and shows non-linear dependence on strain, although the total cavity density, i.e., the sum of the fine and large cavity densities depends linearly on strain. Analysis of cavity size distributions also reveals the formation and growth laws of the large cavities.