Why does \typearea affect page/paper size?

Question

I'd like to use the \typearea package in order to properly calculate the size of the text block. However, invoking the package sets the paper size at A4, and I want to use a smaller size. Here's a MWE, commenting out the \usepackage...{typearea} line results in the correct paper size.

%!TEX TS-program = xelatex
%!TEX encoding = UTF-8 Unicode

\documentclass[12.5pt,headsepline]{book} 

\usepackage[paperheight=7in,paperwidth=4.25in]{geometry}
\usepackage[utf8]{inputenc}



\usepackage{fontspec,xltxtra,xunicode}
\defaultfontfeatures{Mapping=tex-text}
\setromanfont[Mapping=tex-text]{Georgia}
\setsansfont[Scale=MatchLowercase,Mapping=tex-text]{Gill Sans}
\setmonofont[Scale=MatchLowercase]{Andale Mono}


\usepackage[fontsize=12.5]{scrextend}
\usepackage[Conny]{fncychap}

\usepackage{scrextend}

%\usepackage[BCOR=10mm,DIV=calc,pagesize]{typearea}

\usepackage{anyfontsize}


\title{Page Size Problem}
\date{}


\usepackage{kantlipsum}
\begin{document}
\mainmatter
\chapter{Chapter title}
\section{Section title}
\kant[1-15]
\end{document}

Answer 1

It is not possible to use both packages geometry and typearea to set the page layout.

Package typearea knows an option to set the paper size: paper=<width>:<height>. So you could use

\usepackage[pagesize,paper=4.25in:7in,BCOR=10mm,DIV=calc]{typearea}

But note that your fontsize is to large for this small paper.

Answer 2

You can't use the packages geometry and typearea in the same document. There is currently (Jan. 2016) a beta of the komascript bundle which includes a kind of coexistence, but you probably do not use the beta, do you?

If you describe what you whish to do, maybe there is another solution. By the way, why are you using book as a documentclass and scrextend-package, instead of scrbook?