How to adjust brace size for nested braces?

How can I adjust the size of the outer braces to make the following equation neater?

\documentclass[a4paper]{scrreprt}
\usepackage{amsmath}
\begin{document}
$M = \left\{ x \in X : \begin{cases} p(x) > 0 \\ q(x) > 0 \end{cases} \right\}$
\end{document}


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\documentclass{article}

\usepackage{amsmath}

\setlength\delimitershortfall{0pt}
\begin{document}

$M = \left\{ x \in X : \begin{cases} p(x) > 0 \\ q(x) > 0 \end{cases} \right\}$

\end{document}


As shown that is a global setting, it can be set locally within a single expression (or a single {} group) if needed. Also you can experiment with different lengths 5pt is the default.

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What precisely does the delimiter shortfall measure? –  Niel de Beaudrap Sep 25 '12 at 21:44
the delimiters are stretched to be at least \delimitershortfall the size of the expression in the content so by default they may be up to 5pt less than the size of the content a similar parameter is \delimiterfactor which defaults to 901 delimiters must be at least \delimiterfactor/1000 the size of the content. –  David Carlisle Sep 26 '12 at 0:12