Audio settings for each?j
First of all, CBR is not really constant, but varies in small amounts around the indicated bit rate, depending on the footage.
So, even at 100 CBR, one GOP may be encoded at 98 and the next GOP at 102.
Second, with GPU on in CS5, the default encoding is with MRQ.
Both these factors could cause a difference in the resulting file size, although I agree the difference is pretty large, but that may be caused by different audio settings.
The Audio is in both case MPEG-1 Layer II 192 KBit, Stereo, 16 Bit, 48 KHz.
Both files, CS4 and CS5, are rendered with the same Export-Template in Video and Audio.
I rendered multiple times in CS4 and CS5 - the file size was in the CS4-renderings always the same and in the CS5-renderings always the same - bit for bit. In CS5 there was no difference in file size with or without GPU activated.