XviD stats file

[mrpf]

I was just wondering how does xvid calculate the size for the second pass based on the 1st pass stats.

For instance 1st pass stats:
i 2 858 0 0 15515 3289
b 4 0 858 0 3212 1557
b 4 0 858 0 2874 1413
p 2 57 799 2 12858 2279
b 4 0 858 0 4074 1770
b 4 0 858 0 5304 1976
p 2 457 401 0 19269 3208
b 4 0 858 0 1686 689
b 4 0 858 0 1416 681
p 2 18 827 13 11150 1768

The size of the file is 196 kb and the frame sizes taken from the avi for those first 10 frames are:
Frame 1 : 15515
Frame 2 : 16063
Frame 3 : 2874
Frame 4 : 7
Frame 5 : 23336
Frame 6 : 5304
Frame 7 : 7
Frame 8 : 12829
Frame 9 : 1416
Frame 10 : 7

Now suppose that I want to calculate each of those frame sizes for a file size of 108 kb. Any idea what formula is used and how I can accomplish that? In this particular instance the values Im trying to get are listed below:

Frame 1 : 11413
Frame 2 : 10278
Frame 3 : 1737
Frame 4 : 7
Frame 5 : 13332
Frame 6 : 3480
Frame 7 : 7
Frame 8 : 5989
Frame 9 : 782
Frame 10 : 7

Any help is appreciated,
Paul