[dmalinovsky]

The reason for the big margin with the page number is because you can also display the number of the pages left in the current chapter. In this case, this relative number is displayed below the current page number in the book, so there's no margin spared.

But if you don't use this relative number, negative margin will work.

EDIT: After reading your message again, I realized you mean the margin above the numbers, not below. In this case, the margin can be smaller depending on the page layout and the font size — i.e. if a whole line of text will fit above the numbers.