VIM4 how to reduce backup size?

Hello, I have a Khadas VIM4 with Ubuntu installed on it. I need to make a copy of the OS in order to transfer it to another device. For this I use curl -L -OJ. The size of the image is more than 15 GB, although previously the images were about 8 GB. I defragmented the /dev/rootds drive, but that didn’t work.
The df -h command shows:

tmpfs           796M   11M  785M   2% /run
/dev/rootfs      29G  5.1G   24G  18% /
tmpfs           3.9G  4.0K  3.9G   1% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           3.9G  8.0K  3.9G   1% /tmp
tmpfs           796M  104K  796M   1% /run/user/1000

Command cd / && sudo ncdu shows "Total disk usage: 7.8 GiB "

Why has the size of the image grown so much and how can I reduce it?

Hello @bittraid

@hyphop will help you about this issue.