You mean between the bottom of your sig and the bottom of the post window? That's because the stuff on the left side (your username, avatar and other info) takes up vertical space that forces the post window to be a minimum size vertically. Just post really long posts, or get rid of your avatar, and you'll be golden.
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Then I heard the voice of the Lord saying, "Whom shall I send? And who will go for us?" And I said, "Here am I. Send me!"
-- Isaiah 6:8