300 * 500 and 2000 * 2500 have different aspect ratios (0.6 and 0.8). Because you used background-size: cover, the image was resized down in the same aspect rat

300 * 500 and 2000 * 2500 have different aspect ratios (0.6 and 0.8). Because you used background-size: cover, the image was resized down in the same aspect ratio (to not stretch it), and cropped to fit the container, and your screenshot proves it.