Changing book class from one to two sided changes size of footer

Question

I have a larger document (my PhD thesis) which I need to change from one sided layout to two sided layout for publication. However, when simply changing the option of the documentclass book, I suddenly get a lot of underfull vbox badnesses, whose origin turns out to be a change of the size of the footer. I don't get it, why this happens, since here just a change in left or right margins should occur.

MWE

\documentclass[12pt,oneside,a4paper]{book}
%
\usepackage{blindtext}
%
\begin{document}%
%
\blinddocument
%
\end{document}

Output with oneside option:

Output with twoside option:

So there is obviously a change in the margin size between page number and text...

Any ideas?

Answer 1

the margin size is unchanged, although with twoside the body text is shifted horizontally, depending on whether it's a right-hand or left-hand page.

the answer is at the very end of the file book.cls:

\if@twoside
\else
  \raggedbottom
\fi

the reason for this is so that the bottom baseline of full pages is uniform throughout the book. this looks better. it doesn't matter when pages aren't facing one another.

if you really don't care that facing pages don't match, you can always specify \raggedbottom. this can be done in the preamble, or anywhere in the text; it will take effect whenever it is first encountered. and if you want to set set it back, issue the command \flushbottom.

this is also addressed in the question twoside breaks spacing, so this may be a duplicate.