I'm quoting this answer from stackoverflow:
For debugging page layout, Peter Wilson's layouts package is the way to go. Here's an example for visualising the page design and seeing the dimensions used for it
\documentclass[a4paper,12pt]{report}% I didn't add the twoside option in order to only display the right page layouts
\usepackage{layouts}
\usepackage{geometry}
\begin{document}
First you see the default page layout:
\currentpage
\pagedesign
\clearpage
\newgeometry{text={155mm, 24cm}, top=25mm}
Now you can check the page layout you passed to geometry:
\currentpage
\pagedesign
\clearpage
\newgeometry{inner=16.5mm,outer=11mm,top=25mm}
and then check the possible layout you expected (I'm assuming you're using A4 paper)
\currentpage
\pagedesign
\clearpage
\end{document}
As you can see (from the first layout displayed), the default page layout for report (and the other default classes), as @egreg pointed out, does define an outer margin larger than the ineer one.
Since you jest passed to geometry the format for the body (with the text={<textwidth, textheight>} option), it doesn't change the page's default, just adjusting itself to it. In order to achieve what you expect, you must tell geometry what is the inner and outer margins lengths you expect. You can do that both with inner and outer options or with left and right (or lmargin, rmargim, check geometry doc, p. 9)
EDIT: Just updating, following @daleif's comment:
It's considered good typographical practice to use larger outer margins and smaller spines — for the sake of tradition or just to provide the reader a space for personal notes. There's a a wonderfully complete answer explaining that, by Yannis Lazarides (Actually, that question is closely related to yours).
twosideis to have the wider margin on the outside. – egreg Mar 12 '12 at 09:40However, let's clearify the problem: The margins for recto and verso pages differ in my report. Specificly, the space between the left page border and the text is different than the space between the opposite one.
Problem is that this difference is incorrectly applied on recto and verso pages.
One would expect that the left space is larger on the recto page. However, the uneven page numbers (recto) have a smaller left margin compoared to the right. This should be opposite.
– significant Mar 12 '12 at 11:53