剛剛在wiki看到的
'Lossless audio compression allows one to preserve an exact copy of one's audio files, in contrast to the irreversible changes from lossy compression techniques such as Vorbis and MP3. Compression ratios are similar to those for generic lossless data compression (around 50–60% of original size), and substantially less than for lossy compression (which typically yield 5–20% of original size).'

所以大概就是size減半... 實務上應該是這樣沒錯~~ 一般blu-ray影片中, LPCM (沒壓縮過的音效) 5.1 是6.9Mbps, 而壓縮過的lossless dolby TrueHD 5.1大概就只有3~4Mbps