To get the real size used by a rbd image :
rbd diff $POOL/$IMAGE | awk '{ SUM += $2 } END { print SUM/1024/1024 " MB" }'
For exemple :
$rbd info myrbd
rbd image 'myrbd':
size 2048 MB in 512 objects
order 22 (4096 KB objects)
block_name_prefix: rb.0.2c6a.238e1f29
format: 1
$ rbd diff myrbd | awk '{ SUM += $2 } END { print SUM/1024/1024 " MB" }'
14.2812 MB
From Josh in Ceph User mailing list : http://permalink.gmane.org/gmane.comp.file-systems.ceph.user/3684