This paper presents a thorough investigation of a simple method to decrease the dimensions of polymeric microsieves. Pore sizes of microsieves are usually in the micrometer scale, but need to be reduced to below 1 µm to make the microsieves attractive for aqueous filtration applications. In this work polyethersulfone/polyvinylpyrrolidone microsieves were prepared with pores with diameters in the range of 2–8 µm (perforated pores) and a very open internal structure containing many voids. Subsequently, size reduction in terms of pore size and periodicity of the perforated pores was performed by immersing microsieves in mixtures of acetone and N-methylpyrrolidone. Microsieves shrunk because of swelling and weakening of the polymers, and subsequently collapse of the open internal structure. Shrinking typically occurred in two stages: first, a stage where both the perforated pores and periodicity shrink, and second, a stage where the perforated pores continue to shrink while the periodicity remains constant. Microsieves with initial pore diameters of 2.6 µm were reduced down to only 0.2 µm. The maximum isotropic shrinkage was ~35%, which was determined by the amount of voids in the polymer matrix. We propose that to come to higher (nearly) isotropic shrinkage, the amount of voids should be further increased.