Re-encoding Lossy formats with Lossless formats
When data “thrown away” by the encoder, the decoder must still render a full image. The image is just not faithful to the original image, but it is a full image. When you reencode with CRF 0 you are telling the encoder, “I want the output to be identical to the input” and the encoder doesn’t know information was lost on a previous encode. so it must spend bits to encode the “new” image losslessly.