Add an Xpresso tag to one of your objects.
In the X Group window drag in the '48' object.
Click on the red corner and find the scale settings (Coordinates > Scale)
Drag in the other two objects. In their blue corners, click and add the same scale settings.
Anywhere on the XGroup 'desktop' right-click and choose New Node > Xpresso > Calculate > Formula.
On the new Formula node which will have appeared, click on the blue corner and choose Value.
In the attributes window, check the Use Portnames checkbox
In the Formula text field type the following:
From the '48' object's scale port (blue circle), drag a line to the Formula's Value1 port.
From the Formula's output port drag another line to the scale property of your smaller box.
Highlight and copy the Formula node.
Change the formula's text to be
Drag a line from the '48' object to this Formula's value1 and connect up the output in the same way.
And that's it. Your smaller object will retain the same scale relationship no matter what size you put in the largest object.
If any of this is unclear, I'm preparing a tutorial which partly deals with simple Xpresso set-ups like this. Stay tuned.