I'm not sure, are we talking bloated stars or these big "circles" around the center of the image?
Dew/ice on the lens comes to mind. If so, a dew heater can prevent that from happening. Another possibility is a less than perfect lens. If that's the issue, you could try a UV/IR filter to prevent bloated stars.
Are there any light sources close to the scope? Scattered light might enter the scope and make its way to the sensor.