Armbian is written to a µSD card as a small file (a gig or two if I recall). If the card is 64gigs, the image is still just a few gigs written to that card. When booted on the device, though, it automatically expands the file system to the full size of the card.
How exactly does it do that?
And...
How do I do the reverse?
Take the file system from a working armbian install and create a small image that when written to a µSD card, will stay small, but then auto-expand when booted on a device?