Luk Chai was never going to be a large bull given how small his parents are. Sabai seems to be following a similar growth trajectory and will no doubt be similar in size to Luk Chai.
Together, Luk Chai and Sabai were also the two lightest calves at birth:
Birth Weights of Elephant Calves in Australian Zoos
Gung’s Calves:
1.0 Luk Chai born 2009 (Thong Dee): 96kg
0.1 Tukta born 2010 (Pak Boon): 120kg
1.0 Sabai born 2016 (Thong Dee): 101kg
1.0 Jai Dee born 2017 (Pak Boon): 130kg
Bong Su’s Calves:
0.1 Mali born 2010 (Dokoon): 111kg
1.0 Pathi Harn born 2010 (Porntip): 116kg
1.0 Ongard born 2010 (Kulab): 142 kg
1.0 Man Jai born 2013 (Dokoon): 131kg
1.0 Sanook born 2013 (Num Oi): 131kg
Putra Mas’ Calves:
0.1 Willow born 2016 (Num Oi): 103kg
0.1 Kanlaya born 2018 (Porntip): 120kg
You can see Gung's only decently large calves, were born to Pak Boon, who's rather tall and stocky herself. Tukta was an unusually large female calf, and had a similar stature to her mother.
Bong Su managed to sire the biggest calves with some of the shortest females! Kulab is Melbourne's tallest cow now, but at the time of Ongard's birth she was tiny. Dokkoon's Melbourne's smallest cow, yet birthed the equal second heaviest calf.
Putra Mas is apparently a tall male too; although not as tall as Bong Su!