Hello Lee D,
Thank you for your swift reply. Actually, the document setting is to 420x520, which is 2X the desired final resolution. However, all graphics and elements in the document are vector, so that should not matter, right? If I switch to pixel view, the quality is also inferior to the quality I got from saving a larger image and later resize externally. My guess is that small resolution export pays less attention to details somehow - maybe export with much larger size and resize to final user resolution (using interpolation) could solve this problem. That comes with some overhead memory and computation time, of course. But my guess is most users will gratefully invest this - unless they work with insane high resolution art, so maybe make this an option?
Any other thing I can do to help - I could try and upload the designer file?