As I understand it...
When I write the OpenWrt image to the SD card using the dd command, I am effectively copying a pre-configured filesystem structure and operating system onto the card. This image has a fixed size, which is usually much smaller than the capacity of the SD card, especially if using a large one like 32GB.
There are bits of information on using tools to change the partition sizes etc but I can't use those unless it's a one time thing because from remote, I would not dare messing with partitions.
Again, from the little I understand, I should be changing some partition stuff on the sd card after it's written but before putting it into the router. Confusing when you've not done something like this before.