I just found that unchecking ‘Preserve Appearance from Layout’ in the Image panel in the EPUB Export Options solves the problem partially. All the images now change relative to the page width. But cropping is lost, which forces me to adjust the size of some images in Photoshop. Most images luckily have the right proportions, and were not cropped. Is this a bug of InDesign or do I do something wrong? At 150 dpi everything looks fine, but then the size of my epub file is much larger.