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{equation}
\begin{cases}
p = -\frac{1}{\chi}\text{div}(\mathbf{u}),\\
\rho \frac{\partial^2}{\partial t^2} = - \nabla p.
\end{cases}
\end{equation}
I need it the same size as this -
\begin{equation}
p = -\frac{1}{\chi}\text{div}(\mathbf{u})
\end{equation}