i just experienced a weird thing, not really a problem, but i´m curious to know how this can be

i´ve got a movie (avi, DivX502, AC3), but the size was too big for a CD-R, so i thought i´d use virtualdub to split it in two parts

but then it became weird,
i didn´t even need to split,
all i did was to open it in virtualdub, set video and audio to direct stream copy and clicked save as avi
and the resulting file was much smaller than the original, it had shrinked from 911MB to 603MB

i´ve watched both, the original and the re-saved copy, there seems to be no difference at all, i just don´t understand how a direct stream copy can shrink a file by 30%?

any ideas?