Can a glass plate act like a lens?
The box.obj file has no vertex normals, and by default Mitsuba will generate smooth normals for OBJ files that don't specify their own normals. This creates the magnification effect: the box with smooth normals forms a convex lens!

By adding this line to the box object in the scene file:

&lt;boolean name="faceNormals" value="true"/&gt;


I got results that look equivalent to yours: