from Mobac Help:
The output format of an "atlas" for Garmin Custom maps is one KMZ file per layer containing all maps (max 100) as seperate JPG image files. The JPEG compression rate can be specified for each map/image using image format selector in the custom tile processing section. The Garmin Custom Map format defines a maximum image size of 1024x1024 pixels. If a map is larger it will be automatically scaled down to fit into this size. You can prevent scaling when setting the max map size in the settings dialog to 1024.