Bimber Featured Entries grid shows thumbnails in different sizes

Hi, my theme has issues with thumbnails in the header/featured entries section: in the bimber theme configuration, the thumbnails of the featured entries show in identical size. But when opening the page as a normal viewer, the thumbnails vary in size. How can I fix this?

See screenshot with editor (left) and user view (right): http://prntscr.com/w37t4p