» What does the error in COSMOSWorks "Mesh and Model volumes are
» significantly different" mean and how do I correct it?

Tets need to be close to a pyramid shape. Flattened Tets, have a poor aspect ratio (height/base) and will cause this error. Twisted Tets will also cause this error. Reducing the tet size can help. I would suggest mesh control in tight areas and prevent a huge solution matrix.

If your analyzing thin objects, you may want to try shell elements.