1600 x 2560 px is not a round number somebody invented; it is the pair KDP publishes as the ideal cover dimension, and it encodes a 1:1.6 ratio, which is the minimum height-to-width ratio KDP accepts. That single fact explains most cover rejections: designers export square or 16:9 artwork, upload it, and the ratio check fails before the file size even matters. So the resize step is doing two jobs at once — hitting the pixel target and landing on the ratio. Two constraints bracket it. Below, KDP's minimum is 1000 x 625 px, so a 1600 x 2560 cover has room to spare. Above, nothing may exceed 10,000 px on either side, which only matters if you are starting from a much larger master. Resize down, never up: stretching a 900 px cover to 1600 px adds pixels and no detail, and at that point the cover is technically compliant and visually soft. Skwish resizes to exact dimensions with the aspect ratio locked to 1:1.6 and compresses under the 50 MB cap in the same pass.