Why don't I post what's extra in the English versions and someone else can do the same for the Japanese ones. Then we can compare...
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Vol. 1 (english):
1 pg: Artist's Comic - introduces the manga crew.
1 pg: Assistant Page - shows Dorothy in a Big O armor.
1 pg: Gallery of covers of the original comics.
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Vol. 2 (english):
10 pgs: Paradigm Gallery - sample artwork with comments below
7 pgs: Interview #1 with Big O creators
1 pg: Assistant Page - shows Dorothy holding a mop w/ Instro
4 pgs: Mini comic - Dorothy lulls a tire Roger to sleep
1 pg: Gallery of covers of the original comics.
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Vol. 3 (english):
1 pg: Artist's Comic - artist is tired from work.
1 pg: Assistant Page - shows Dorothy attacking Beck with knife
1 pg: Gallery of covers of the original comics.
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Vol. 4 (english):
1 pg: Gallery of covers of the original comics.
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Vol. 5 (english):
1 pg: Artist's Comic - artist is wondering around.
1 pg: Assistant Page - shows Dorothy riding a scooter
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Vol. 6 (english):
1 pg: Artist's Comic - artist is compares manga with anime.
1 pg: Assistant Page - shows Angel holding a gun
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So it looks like after Vol. 2, they got totally lazy!

You think they will put out an extra's issue? I've seen that for some Manga, like Appleseed.