The top item is always the larger of the items. Clicking on the 'Something bigger' button will replace it. Clicking on the smaller item (on the bottom) will cause it to become the top item.
The bottom item is always the smaller item, and clicking on the links on the right hand side (various smaller items) or on the 'Something smaller' button will cause that item to replace the bottom item.
The bottom item is always the smaller item, and clicking on the links on the right hand side (various smaller items) or on the 'Something smaller' button will cause that item to replace the bottom item.