I wish to understand.
There are Europe maps 1G and 2.6G - what is missing in 1G that is in 2.6G?
Is more detail in single country maps? What detail?
There are various map combinations eg. USA can be 1G and there is NE, SE, South etc. which add up to 5G.
What are the differences?

Mostly the differences are Advanced Lane Guiding and Text To Speech files.
Coverage of streets is the same. So do not worry your pretty head 'bout that.

The topics containing the maps always have a informative table which shows the differences. Doesn't hurt to study that.