To clarify, and also for anyone who comes across this post in future, I'm going to explain like they know nothing about this.
Creating the SD card for the Pi divided the card into different "sections" (Partitions). Windows can't see the main Linux (Raspbian) partitions. it can only see one tiny FAT system partition. All the space is still there, but Windows just can't see it. If you want to use the card for something else in the future, just format it in the same way you would format it ready for making the bootable card for the Pi, and it should be back to normal. All the partitions are accessible from the Pi.
I hope this, combined with Mahjonng's post above, explains it to anybody interested.
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