Wrong "disk size" after making qemu-img convert -f vpc -O qcow2

[kermu]

After making qemu-img convert -f vpc -O qcow2 Ubuntu.vhd Ubuntu.qcow2 I got incorrect "disk size" in qcow2 image file.

~/qemu-img info Ubuntu.vhd
image: Ubuntu.vhd
file format: vpc
virtual size: 7.87 GiB (8455200768 bytes)
disk size: 23.2 GiB
cluster_size: 2097152

~/qemu-img info Ubuntu.qcow2
image: Ubuntu.qcow2
file format: qcow2
virtual size: 7.87 GiB (8455200768 bytes)
disk size: 6.65 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false

I don't know where is problem with wrong calculation "disk size" (23.2 GiB for Ubuntu.vhd versus 6.65 GiB for Ubuntu.qcow2 after conversion)
In consequence for mentioned above, image Ubuntu.qcow2 not boot anymore, because grub get into recovery mode
It's bug in qemu-img ?

Christopher