I think you may have keyed the vertices of a mesh.
In mesh deformation animations, vertices always interpolate in a straight line between positions. So if you rotate the verts, they travel in a straight line and in the direction that the total shape becomes smaller before returning to the resulting size.
What you probably want to do is rotate the parent bone instead; not move the vertices. Then it won't have that "scaling effect".
To do this, make sure you select the bone (not the image, or the vertices) before using the Rotate tool.
Animating via bones is the standard way to animate in Spine. This is why it's called "skeletal animation".