Object scales down when I try to rotate it using a armature/ bone set up

I have a model of a fighter with a swing wing set up which I am trying to animate using a rig system. I have set up an armature with a bone on each wing. When I grab the bone, the wing moves fine with it. When I rotate the bone, the wing scales down. I want to be able to rotate the wings on their Z axes but this scaling effect is happening regardless of the axis I rotate on.
I recreated the effect on a simple blend file which I’ve attached so I’m sure it’s a simple setting I’ve missed but it’s doing my head in trying to find it! I’ve applied all transformations so I don’t think it’s that. Could anyone take a quick peek and see if they can help? All I am trying to do is rotate the wings 120 degrees on the Z axis!
Many thanks in advance.

The test file is here: https://drive.google.com/file/d/0B2WqHg3IE2c6UmtOYnFQTWNkOEE/edit?usp=sharing.

All good, I had put the wings into the root vertex group as well as their separate ones. Cheers to JC for pointing that out. Knew it’d be simple!