There are several settings in Android to manage the storage-location of an app. First, you can set the preferred installation to external sd-card. Second, you can (and may have to) select for each downloaded app to get moved to the external sd-card.
If you move MapFactor to the external sd-card before downloading the maps, the downloader will then chose a folder on the external sd-card to store the maps.
Sometimes, even if you have set up the preferred location on the sd-card for your new downloaded app, Android is using the internal sc-card first, i don't know why, but it sucks. So you have to go to the setting for each app and make sure it has been moved to the external sd-card.
@wunderlicht I'm not sure if this is applies to Android 4 (ICS). I think with ICS there is no longer the possability to change the apps install location.
With Android 4.1.2 on the LG-P710 I simply can move the application to the external card by opening the systems configuration menu, going to "applications", clicking on the navigation app, and then chosing the option "move to card". I hope the translation to English works, because I have a German installation.