Devs add size option probably because not everybody have 16 or more gigabytes of RAM. The bigger size of map preview you want to generate the more RAM you need. For example: by generating map size 1, you generating only 9 hubs/regions(3x3) where hub/region is 512x512 blocks in size or 32x32 chunks, where chunk is 16x16 blocks. By generating preview size 2 you generating only first 25 regions(5x5) from center of the map, size 3 - 49, size 4 -81 and size 5 -144 regions.