I did some testing with a WindowsXP guest, that I have and could test on.
It seems that this behavior is not present at the beginning. But at the moment we create a snapshot it is starting to write on top of the current size. So, it is like this:
 1. Image is:

Code:
qemu-img info WindowsXP.img 
image: WindowsXP.img
file format: qcow2
virtual size: 15G (15728640000 bytes)
disk size: 13G
cluster_size: 65536

I write some files and it doesn't become more than that.

 2. I delete some files, then write again and it doesn't changes size.
 3. I create a snapshot:

Code:
qemu-img info WindowsXP.img 
image: WindowsXP.img
file format: qcow2
virtual size: 15G (15728640000 bytes)
disk size: 13G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         test                      0 2012-07-17 09:52:25   00:00:00.000

4. I write something with a size of 587MB and it becomes larger (with 587MB)

Code:
du -sm WindowsXP.img 
14102   WindowsXP.img

5. I delete it and then write it again. It becomes more larger... (again with another 587MB)

Code:
du -sm WindowsXP.img 
14703   WindowsXP.img

6. I delete it and then write it again. It doesn't change this time.

7. I write a copy of it. It becomes larger (with 587MB)

Code:
du -sm WindowsXP.img 
15309   WindowsXP.img

8. I delete it and then write it again. It doesn't change this time.

9. I write a copy of it. It becomes larger again (with 587MB)

Code:
du -sm WindowsXP.img 
16010   WindowsXP.img

10. I write another copy of it and it stays the same.

Code:
du -sm WindowsXP.img 
16010   WindowsXP.img

11. I write another copy of it and becomes larger again.

Code:
du -sm WindowsXP.img 
16913   WindowsXP.img

Code:
qemu-img info WindowsXP.img 
image: WindowsXP.img
file format: qcow2
virtual size: 15G (15728640000 bytes)
disk size: 17G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         test                      0 2012-07-17 09:52:25   00:00:00.000

12. Create a snapshot.
13. I write a copy of it. It becomes larger again (with 587MB)

Code:
du -sm WindowsXP.img 
17572   WindowsXP.img

14. I delete both of the snapshots.
15. I delete the file (in guest with the size of 587MB) and write it again. No change in size.
16. I delete the file again and write it again. No change in size.
17. Create a snapshot.
18. I delete the file again and write it again. No change in size.
19. Delete the file.
20. Create a snapshot.
21. Write the file again. No change in size.


 Well from all this, I can conclude that most probably:
 1. The problem occurs only when there is an internal snapshot present.
 2. The problem is not "by design" because the behavior is not consistent (for example, 13. and 21. should be with the same result, but they arent't)..


At the end of the day, after these procedures (4 creations of snapshots, 2 deletions and some writing and deleting of internal guest files) the result is this:

Code:
qemu-img info WindowsXP.img 
image: WindowsXP.img
file format: qcow2
virtual size: 15G (15728640000 bytes)
disk size: 17G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         test2                     0 2012-07-17 14:55:17   00:00:00.000
2         test3                     0 2012-07-17 15:27:43   00:00:00.000

and

Code:
ls -lsh WindowsXP.img 
18G -rw------- 1 libvirt-qemu kvm 18G Jul 17 15:43 WindowsXP.img