Efficiency of 2 pass on x264

[kaz219]

I'm trying to make some tests with x264 2 pass encoding. Here is what I have after the first encoding pass:

x264 [info]: slice I:25    Avg QP:27.36  size: 18372  PSNR Mean Y:40.19 U:43.41 V:43.69 Avg:41.03 Global:40.64
x264 [info]: slice P:2401  Avg QP:29.66  size:  5245  PSNR Mean Y:38.27 U:42.25 V:42.56 Avg:39.24 Global:38.45
x264 [info]: slice B:1265  Avg QP:31.48  size:  1690  PSNR Mean Y:37.55 U:42.01 V:42.43 Avg:38.60 Global:37.80
x264 [info]: mb I  I16..4: 33.2%  0.0% 66.8%
x264 [info]: mb P  I16..4:  7.9%  0.0%  9.3%  P16..4: 49.6%  0.0%  0.0%  0.0%  0.0%    skip:33.2%
x264 [info]: mb B  I16..4:  0.6%  0.0%  1.5%  B16..8: 26.0%  0.0%  0.0%  direct: 2.5%  skip:69.5%
x264 [info]: final ratefactor: 26.31
x264 [info]: PSNR Mean Y:38.034 U:42.176 V:42.523 Avg:39.030 Global:38.230 kb/s:823.13

encoded 3691 frames, 2.85 fps, 824.20 kb/s

And here is what I have after the second pass:

x264 [info]: slice I:25    Avg QP:28.32  size: 16338  PSNR Mean Y:39.60 U:42.98 V:43.28 Avg:40.48 Global:40.17
x264 [info]: slice P:2401  Avg QP:29.53  size:  5386  PSNR Mean Y:38.37 U:42.27 V:42.58 Avg:39.34 Global:38.91
x264 [info]: slice B:1265  Avg QP:31.60  size:  1741  PSNR Mean Y:37.55 U:41.96 V:42.38 Avg:38.60 Global:38.18
x264 [info]: mb I  I16..4: 35.3%  0.0% 64.7%
x264 [info]: mb P  I16..4:  7.6%  0.0%  9.8%  P16..4: 49.2%  0.0%  0.0%  0.0%  0.0%    skip:33.4%
x264 [info]: mb B  I16..4:  0.6%  0.0%  1.6%  B16..8: 25.5%  0.0%  0.0%  direct: 2.3%  skip:70.0%
x264 [info]: PSNR Mean Y:38.100 U:42.166 V:42.519 Avg:39.094 Global:38.654 kb/s:842.19

encoded 3691 frames, 2.36 fps, 843.26 kb/s

Is it normal? I mean, with ffmpeg and XviD, a two pass encoding doesn't look like this at all, since the first pass has a very high bitrate. Here the first pass' bitrate is even lower than the second pass.

If anybody has also feedback about 2 pass on x264, I'm interested.

cheers,