Polyester resin has about a 6% volume shrinkage on cure compared to less than 3% for epoxy. How the shrinkage shows up in a part can be pretty complex. It may go unnoticed in a wet-laid or vacuum-bagged part but can cause insurmountable headaches in a closed mold where shrinkage results in separation from the mold surface and pitting. Fiber orientation, part thickness, fiber volume and cure temperature are all important factors.