Koken publishes images using default values for quality and sharpness that are recommended for most publishers. You can see these values (and make edits to them) by clicking on Settings at the top of Koken then Image Publishing in the left column. In the content area will be an option for Quality / Sharpness. Uncheck the Use default settings checkbox and the interface will expand to show this:
Each row controls the quality and sharpness for a particular image preset size. Left slider is quality, right slider is sharpness. Make any adjustments you'd like, then click the Save settings button at the bottom to save. Next time images are requested on your site new versions will be generated using your edits.
When a custom image size is requested by a theme, Koken will check to see which pair of presets the image is larger/smaller than, then use the quality/sharpness values of the smaller preset. For example, if you were requesting a custom image 600px wide, Koken would use the quality/sharpness values of Medium (480px).
To revert your changes and return to default values, check the Use default settings checkbox then save again.