I managed to resize the overlay.
The tricky (nasty) part is that there's two filesystems in the second partition. There's a squashfs and right after the squashfs a hidden f2fs. A loopmount with an offset is needed. One needs to install losetup and use it to display the offset on the running system, but I don't know how to tell the offsetz from a given image directly).
Then do the same loop mount on a linux system and do a resize.f2fs.
Why are two filesystems (squashfs and f2fs) put in the same partition? Why doesn't the f2fs have it's own partition?