From Leopard Gecko Wiki
The Giant morph was developed by Ron Tremper in 2000. The Giant morph is thought to be a Co-Dominant trait. There are three phases of the Giant morph, Normal, Giant, and Super Giant. The Normals do not express the trait at all. The Giants express the trait and are the heterozygous form. The Super Giants are the homozygous form of the morph and are very large.
Ron Tremper's original Giant, Moose was a Super Giant. He reached a whopping 156 grams. When The Herpetoculture of Leopard Geckos came out, Moose was the largest Leopard Gecko ever. On October 9, 2007, Moose unexpectedly died at age 6 weighing 154 grams.
With the size difference between Super Giants, Giants, and Normals, one would think it would be difficult to succeed with a breeding combination like a Normal with a Super Giant. Currently, there have been no problems with breeding a Normal to a Super Giant or vice versa.