Turns out, that CKFinder will resize any uploaded image to 1600x1200 if the image is larger than that. This is doubtful feature of CKFinder (see http://docs.cksource.com/CKFinder_2.x/Developers_Guide/PHP/Configuration/Images) that is also enabled in default configuration.
Solution
Don't let CKFinder to change uploaded images.
Related Tasks
INP-1456 - Getting issue details... STATUS