Comment 2 for bug 565659

What image format is the base image?
I just tested with qcow2 format and increased the size from 6g to 10g just fine (of course the file system size remained the same)
 kvm-img create -b Lucid_64_Base.qcow2 -f qcow2 test_img.qcow2 10G

If you don't specify a format I think raw is used:
kvm-img create -b Lucid_64_Base.qcow2 test_img.qcow2 10G
Unknown option 'backing_file'
qemu-img: Backing file not supported for file format 'raw'

Try using kvm-img info [imagefile]
to see what format is being used.