this is expected, if you use the combined image, it'll rewrite the partition table, and their sizes.
zcat the ext4 root fs to /dev/mmcblk0p2, and copy the kernel file to /dev/mmcblk0p1, adapt the boot loader.
(the is very high level, I don't own a NanoPi, it's how it's done on x86).
if you'd have a primary and secondary openwrt partition you could do something like