Offset setings defines distance between an object and its container. When offset value is positive the object becomes smaller than the container and when it is negative the object becomes large.
By default when offset is equal to 0 container and object sizes are the same. But as soon as you change offset settings container and object sizes becomes different. To see this difference you need to switch Container/Object options with corresponding radio buttons. This option doesn't make any changes in the geometry but only shows the real size either of container or the object.