Text is smaller in fractions in multiline equations?

The text size shrinks in a multiline equation when I use fractions. How can I keep it the same as the non-multiline equation size?

$$\begin{cases} p = -\frac{1}{\chi}\text{div}(\mathbf{u}),\\ \rho \frac{\partial^2}{\partial t^2} = - \nabla p. \end{cases}$$

I need it the same size as this -

$$p = -\frac{1}{\chi}\text{div}(\mathbf{u})$$

Using \dfrac instead of \frac will force the fraction to display at the "usual" size.

Hence using this code will work:

$$\begin{cases} p = -\dfrac{1}{\chi}\mathrm{div}(\mathbf{u}),\\ \rho \dfrac{\partial^2}{\partial t^2} = - \nabla p. \end{cases}$$

Edit: As suggested by Manuel, using \dcases is a better alternative

The code becomes

$$\begin{dcases} p = -\frac{1}{\chi}\mathrm{div}(\mathbf{u}),\\ \rho \frac{\partial^2}{\partial t^2} = - \nabla p. \end{dcases}$$
• May be better use dcases instead of cases? – Manuel Nov 29 '15 at 13:28

You can use the dcases environment provided by mathtools. Note also some improvements to your code.

\documentclass{article}
\usepackage{amsmath,mathtools}

\DeclareMathOperator{\Div}{div}
\newcommand{\vect}[1]{\mathbf{#1}}

\begin{document}

$$\begin{dcases} p=-\frac{1}{\chi}\Div\vect{u}, \\ \rho\frac{\partial^2}{\partial t^2}=-\nabla p \end{dcases}$$

\end{document}

A further improvement for the partial derivative: with

\newcommand{\pder}[3][]{%
\frac{\partial\if\relax\detokenize{#1}\relax\else^{#1}\fi#2}
{\partial#3\if\relax\detokenize{#1}\relax\else^{#1}\fi}%
}

in the preamble, you can use

\pder{f}{t}
\pder[2]{f}{t}
\pder{}{t}
\pder[2]{}{t}

and your case would be the last one.