This is normal behavior. The quality measurement function is designed to work with multiple types of sensors. For this reason, frame size is one of the parameters influencing quality rating. This is intended to generate reasonable quality ratings on both small and large sensors. In order to get consistent results, you should decide on standard canvas size for your project and convert all input images to this standard size. This way the quality rating will be consistent. For more detailed explanation, see the topic How exactly is the fingerprint quality calculated.