ediff-wide-control-frame-rightward-shift is a variable defined in `ediff-wind.el'.
Its value is

The rightward shift of control frame from the left edge of buf A's frame.
Measured in characters.
This is used by the default control frame positioning function,
`ediff-make-frame-position' to adjust the position of the control frame
when it shows the full menu. This variable is provided for easy
customization of the default.

You can customize this variable.