Size of braces and parentheses in split environment

All,

The following display equation using a split environment produces  
curly braces and parentheses of different sizes in each line of the  
expression. I would like both lines to have braces and parentheses  
that have the same size as those in the first line.

Your help is appreciated.

  \begin{equation}
\begin{split}
	K_z  &= \frac{1}{\left( \sigma_{x}^{2} + \sigma_{y}^{2}\right)^{2}}  
\: \left(\mathcal{E} \left\{\left( X- \eta_x \right)^4 \right\} +  
\mathcal{E} \left\{\left( Y- \eta_y \right)^4 \right\} \right. \\
	& \quad \quad +\left. 2\,\mathcal{E} \left\{\left( X- \eta_x  
\right)^2 \left( Y- \eta_y \right)^2 \right\}  \right)
\end{split}
  \end{equation}

Thanks in advance,

Raul Martinez

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