Understanding how dd works

[Dan Track]

Hi

I've got a xen vm file called test, if I copy it with dd I get the following
dd if=/opt/xen/test of=/opt/test-vm.img bs=4096
du -s /opt/xen/test = 1934112
du -s /opt/test-vm.img = 26240040

My question is why is the test-vm.img larger in size than the original?

Thanks
Dan