Equation editor allows zooming in and out. This does not influence actual equation size in the document.

To change zoom, select the equation and tab Edit|Equation Object|Open.The equation editing window will open (see pic.2), it allows setting zoom of 25% to 400% through the View menu.

Pic. 2. Equation Editor, Zoom set to 200%

On the other hand, Edit|Equation Object|Edit menu item will activate regular equation editing mode that provides no zoom control.