Open a page with thumbnails via "Edit in Word" displays the image in full size. Saving (wihtout changing anything) resizes thumbnail (in its resolution) to full size.

The markup before and after:

{loremipsum:1}
!Uebersicht-Icon-Anpassung.png|thumbnail!
{loremipsum:1}

goes to:

{loremipsum:1}
!Uebersicht-Icon-Anpassung.png|thumbnail=,height=526,width=1663!
{loremipsum:1}