Scaling a spring mesh to create a morph target that could be used to make the spring appear to compress is not strictly accurate, in terms of the physics of a real spring. Scaling will not only change the length of the spring, it will distort the cross section of the coiled steel as well. If the spring was made by rotating a circular N-gon around an axis, then stretching the mesh will have the unwanted effect of making the spring appear to have been made by rotating an ellipse.
A real coil spring, when compressed or expanded, puts a torsional stress on the coiled rod, effectively twisting it across it's full length. In some cars the suspension is based around a torsion bar, which is effectively an uncoiled spring. To be strictly accurate, the way to create a spring compression morph in animator would be to rotate each N-gon cross section very, very slightly on an axis perpendicular to the spring long axis and the N-gon cross-section plane. Although this would be VERY tedious, it would provide an accurate model of the way the spring expands in overall diameter when compressed, and shrinks when stretched. Perhaps a compromise would be to shift each cross section N-gon a tiny bit in the direction of the spring's long axis, creating an overall change in length but not changing the shape of the actual coil.
