嗯.........你的疑問我也沒辦法完全回答....等待其他人的回答..........
嗯.........對了.....你是用CBR還是VBR啊？
或許是因為你用CBR，固定流量，縱使因為用Joint stereo減少兩個聲道的資料量，但又要維持固定的流量，於是就多採集更多的其他音樂的資料來補足？（我會這麼猜是因為我習慣用VBR-ABR的格式，在提高聲音品質的前提，檔案大小不會變得太大）


另外.......以下是CDex對於這兩個格式的解說，參考看看吧！

Stereo: In this mode, the encoder makes no use of potential similarity between the two input channels. It can, however, negotiate the bit demand between both channels, i.e. give one channel more bits if the other contains silence.

Joint stereo: In this mode, the encoder will make use of a correlation between both channels. The signal will be matrixed into a sum ("mid") and difference ("side") signal. For quasi-mono signals, this will give a significant gain in encoding quality. This mode does not destroy phase information like IS stereo that may be used by other encoders. This setting can be used to encode DOLBY ProLogic surround signals.