If you are allowed to make some minor adjustments you may probably even leave out most of the \cdot commands, which will turn the formula even sleeker ... Just my two cents, Oliver
\starttext
\startformula \frac{g_a}{\theta} = \startcases[left={\left[},right={\right]}] \NC \displaystyle
\startcases[left={\left[},right={\right]},style=\displaystyle] \NC
H_M \cdot z^{1 - \phi} \cdot \frac{\phi - \sigma \cdot \phi}{\sigma - \phi} \cdot \frac{(1 - \phi) \cdot \gamma}{1 - \sigma} - H_S \cdot z^{-\phi} \cdot \frac{\phi - \sigma \cdot \phi}{\sigma - \phi} \cdot \frac{(1 - \phi) \cdot \gamma}{1 - \sigma} + \gamma \cdot H_S \cdot z^{-\phi} \NR \NC \displaystyle
\NC
- \gamma \cdot z^{-\phi} \cdot \frac{\phi - \sigma \cdot \phi}{\sigma - \phi} \cdot z \cdot H_M + \gamma \cdot z^{-\phi} \cdot \frac{\phi - \sigma \cdot \phi}{\sigma - \phi} \cdot H_S \NR \stopcases \stopformula
\stoptext