A little while ago I took a 4:3 1080p lossless Blu-Ray rip of a public domain short film, A Trip to the Moon. Straight off the Blu-Ray it's around 2.5 GB, at 1920x1080. (The left and right are made up of black bars.) 

Regardless of whether ffmpeg supports it (I don't think it does) is it technically possible to losslessly crop the black bars of the video, such that the final size is <=2.5 GB and every pixel within the 4:3 1280x1080 frame is preserved exactly as it appears on the Blu-Ray? When I used ffmpeg's crop commands, the final 15 minute video was 12 GB, because it was lossless and as I understand it it's like saving a JPEG as a PNG - the previous format compressed it well but ffmpeg couldn't crop it and keep the nice compression. The video is AVC in an MKV container.

The only point of comparison I have is that a photo editing tool, Irfanview, has a JPEG lossless crop feature that re-arranges the relevant data within the file so it is neither re-compressed as a new JPEG (which causes quality loss) nor saved at a file size greater than the original size.