As I understand it, part of the reason Tiffany's breasts appeared so soft and 'real' was due to her implants being filled with silicone, rather than saline or other, similar substances. Silicone implants, I'm told, have a consistency that is much closer to actual breast tissue - especially at larger sizes - than other, comparably sized implants.
I'm hardly an expert however. Someone with more knowledge of Tiffany and her career might be able to say more.