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When you set the JPEG compression level to 100%, it doesn't mean that the compressed image will be exactly the same as the original image. Rather, it means that the compression algorithm will try to preserve the maximum amount of detail and color information possible while still achieving some level of compression.
In some cases, setting the compression level to 100% can actually result in a smaller file size than the original image, especially for images with a lot of detail or high levels of noise. This happens because the compression algorithm is designed to identify and remove redundant or unnecessary data, which can sometimes result in a smaller file size than the original image.
When the JPEG compression algorithm applies its various stages of compression to an image, it discards information that it deems less important to the overall image. In particular, the quantization stage discards high-frequency components of the image that are less noticeable to the human eye. This means that some of the fine details and noise in the image may be lost, resulting in a smaller file size.
However, it's important to note that setting the compression level to 100% doesn't always result in a smaller file size than the original image. In some cases, it may result in a larger file size or a file size that is similar to the original image, especially for images with fewer details or low levels of noise. It's also worth noting that JPEG compression is a lossy compression technique, so even at 100% compression, some level of image degradation will occur.