Shape Keys rotation animation is shrinking my mesh?

Hi all,

I have a mesh that is somewhat like a smashed egg. I am using Shape Keys to rotate the mesh 90 deg clockwise. Unfortunately, halfway through the rotation the mesh shrinks in volume. I can see what is happening but I can’t seem to sort out how to prevent it. I need the mesh to remain exactly the same size throughout the rotation. (This is for a mechanical animation). Also, this animation will be exported as FBX for UDK import - will this behavior effect the final product?

Thank you for any help/Advice.

BTW-the gif I linked is animated but drop box will not show as such. so if you download and then preview in your browser the issue I described above should be obvious.

Shapekeys are a linear translation, the vertices are not really rotating around your object even if you used a rotation to reposition the vertices