BaseCamp Map Install VS Mapsource Question

I am new to Base Camp and I notice that when you install a map from Map Installer to your GPS or in my case an SD card it only indicates the available space on the SD card. Whereas Map Source indentifies the number of map segments which is the determining factor for the GPS processor as my GPS can only process up to 4000 map segments. Why is it done this way if the unit has a limit?

Also, I notice that the map install .img file is a full download to the card (ie 1.5GB) so if I save this to my hard drive will I be able to just copy this to another card without re-installing this map from Map Install?