I'm not sure that I would entirely agree with this statement either. A good example would be the Grizzly Xtreme ammunition. ( Grizzly Xtreme)

While it's a solid copper round like the Barnes X bullets, it maintains it's fully expanded diameter after penetration, causing massive deceleration and underpenetration. On the other hand, the Barnes X does not maintain full expanded diameter and the petals fold back, reducing the drag profile and increasing the depth of penetration.

The only way I could think of maintaining the diameter of full expansion without reducing penetration is to reduce the diameter of the fully expanded round?