Comment 4 for bug 565659

"should really raise a warning if ../base-dont-touch.img is smaller than 10GB"
Here you are saying if the forked image is larger than the base image.

"If QEmu doesn't handle a differnent size, even a bigger one, I don't see why qemu-img create -b ... should take a size argument at all."
I just showed that there is a reason why there is a size argument.

If the issue is when the forked image is smaller than the base, I suspect that the issue is that the filesystem was not reduced in size first, kvm-img is not responsible for this. Did you remember to shrink your partitions before shrinking the image size?

Also, you do not need to include a size when using -b with the kvm-image create.
Are you intentionally trying to reduce the disk size? Why? If you are trying to recover space on the host system, you can zero out the free space on the guest (bleachbit can do this) then convert your image using -c to compress the image...then use that image as a base image. Qcow2 images only use up what is needed.