Re-encoding Lossy formats with Lossless formats I was re-encoding videos downloaded from YouTube with Lossless H.264 (using FFmpeg) because I'm experimenting with CRF=0. 

I found that resulting videos are much bigger (1 gigabyte) than the original video (100 megabyte). 

However, from my knowledge of lossy compression, it is basically like lossless compression (plus, it's the same codec) except it throws away lots of minor details that won't be noticed but take up lots of space. Hence, shouldn't I achieve a similar file size if I encode the decoded file using CRF=0 because lots of data are already thrown away? I don't quite understand this.

I use the following command: 

ffmpeg -i 'file' -an -c:v libx264 -crf 0 'out file'
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.