Open Stack

hi i am using cloud-init on Centos6.6 instances.
Issue is instance's disk size is increasing but partition size not
expanding according to flavor size

i have growpart installed.

growpart /dev/vda 1 shows following error
NOCHANGE: partition 1 is 83883776.It can not be grown.

But if i resize its working but not completely.

Actual qcow2 disk is 2G size and allocated is ~1G.

Strange Behaviour:
With flavor disk size of 20G,fdisk shows disk size of 20G, but
partition is still 2G.
If resized to different flavor with disk size of 40G,fdisk shows 40G
and partition is 20G!!
if  resized to larger flavor with disk size 80G,fdisk shows 80G and
partition is 40G !!

So the issue from first flavor continues,if start with 40G disk then
difference will always be 40G

Why is cloud-init behaving like this