Maybe I am just too tired, but I simply can't explain what is going on.
The file
\documentclass[a5paper]{scrbook}
\usepackage{lipsum}
\begin{document}
\lipsum
\end{document}
produces A4 sized pdf pages (when run with pdflatex) with text only in the upper left corner, while
\documentclass[a5paper]{scrbook}
\usepackage{tikz}
\usepackage{lipsum}
\begin{document}
\lipsum
\end{document}
produces the proper A5 pages. Why is that?