Sentence:  The desired symmetry was only achieved by "<WSD>formatizing</WSD>" great masterpieces; and this was why Christian von Mechel had to remove additions to pictures and remedy losses when the Hapsburg paintings were moved from the Stallburg to the new museum in the Belvedere Palace. 
Meaning of the word marked with <WSD> tags:  To trim or add to a picture in order to make it fit into a standard frame size.