From Qcow in Wikipedia:
One of the main characteristics of qcow disk images is that files with
this format can grow as data is added. This allows for smaller file
sizes than raw disk images, which allocate the whole image space to a
file, even if parts of it are empty.
So the file size is really 13MB, but it can grow up to 20GB when data is written to it. Example:
$ qemu-img create -f qcow2 test.img 2G
Formatting 'test.img', fmt=qcow2 size=2147483648 encryption=off cluster_size=65536 lazy_refcounts=off
$ ls -l test.img
-rw-r--r-- 1 carvalho carvalho 197120 Jul 18 09:30 test.img
$ file test.img
test.img: QEMU QCOW Image (v2), 2147483648 bytes
An empty qcow2 file was created. It can hold a 2GB filesystem, but for now it only occupies 197KB in disk.
The "cow" part of qcow2 is an acronym for copy on write, a neat little
trick that allows you to set up an image once and use it many times
without changing it. This is ideal for developing and testing
software, which generally requires a known stable environment to start
off with. You can create your known stable environment in one image,
and then create several disposable copy-on-write images to work in.
To start a new disposable environment based on a known good image,
invoke the qemu-img command with the option -o backing_file and tell
it what image to base its copy on. When you run QEMU using the
disposable environment, all writes to the virtual disc will go to this
disposable image, not the base copy.
If the option backing_file is specified, then the image will record
only the differences from backing_file. No size needs to be specified
in this case. backing_file will never be modified unless you use the
"commit" monitor command (or qemu-img commit).
$ qemu-img create -f qcow2 -o backing_file=test.img test01.img
Formatting 'test01.img', fmt=qcow2 size=2147483648 backing_file='test.img' encryption=off cluster_size=65536 lazy_refcounts=off
$ file test01.img
test01.img: QEMU QCOW Image (v2), has backing file (path test.img), 2147483648 bytes
In your case /var/lib/nova/instances/_base/035db99541e92b5cca93bf18a997d626f512b73d is the backing file. I don't know what is the expected bahavior if you try to use your qcow file without the backing file.
/var/lib/nova/instances in OpenStack documentation:
This directory contains the libvirt KVM file-based disk images for the
instances that are hosted on that compute node. If you are not running
your cloud in a shared storage environment, this directory will be
unique across all compute nodes.
/var/lib/nova/instances contains two types of directories.
The first is the _base directory. This contains all of the cached base
images from glance for each unique image that has been launched on
that compute node. Files ending in _20 (or a different number) are the
ephemeral base images.
The other directories are titled instance-xxxxxxxx. These directories
correspond to instances running on that compute node. The files inside
are related to one of the files in the _base directory. They're
essentially differential-based files containing only the changes made
from the original _base directory.