Please provide the direct link to the feed and which particular item are you referring to.

http://www.xyz.com/magazin/Eintr%C3%A4ge/1613-mieten-vermieten-bhg-st%C3%A4rkt-vermieterrechte-bei-eigenbedarf
http://www.xyz.com/administrator/index.php?option=com_easyblog&view=feeds&layout=form&id=6

EasyBlog's feed import will try to connect to the site at http://www.immonewsfeed.de/news.php?id=1659 to fetch the image. This is the image that is being used on the post and this is why it's fetching the smaller version of the image.

If you want to use whatever that is provided in the RSS, ensure that the option "Get full text from feed" is disabled.