The whole map can't have only 6.8 MB. Usually, GPS maps occupies more that the seize mentioned. However, if indeed the whole map is transfered to the device, probably you are doing something wrong. In this matter, I strongly suggest that you visit the developer's website. From there, you should be able to access the tutorial videos in which it is explained how to install the program, and how to use it as well (the videos can be found on the bottom of the page). Follow the steps described in the tutorial to properly transfer items to your GPS device.
