I’ve read somewhere that Kiwis are in the running for being one of the closest genetic descendants / relatives of the T. rex and if they ever get off New Zealand and are relieved of the island dwarfism pressure…
Definitely not my area of expertise though, purely a fun speculation.
No, the opposite is the case. You can just scale an animal up. But it won't work well; the larger size requires different support structures than the normal size does. Hence, giant humans tend to be in poor health, with circulatory and other problems.
The reason island populations show size changes is that the tiny environment leaves environmental niches open, and animals of the wrong size can radiate into those empty niches without facing competition from animals that are already filling them more effectively. Island dwarfism is common, and so is island gigantism.
Note that New Zealand obviously is small enough for island dwarfism, since it was small enough for island gigantism. The size seems less relevant than the fact that it was isolated by deep ocean.
The two main parts of the country are the 12th and 14th largest islands on the planet: https://en.wikipedia.org/wiki/List_of_islands_by_area
There's four.
One is icy AF, and another contains 95% of the world's deadly organisms.
Though that occurs in other contexts as well: Antarctica (hardly small) with penguins, Africa (dittos) and ostriches, Australia (again) with emus, and South America (again) with the rhea.
There are also flightless island birds, such as the Guam Rail, Henderson crake, Inaccessible Island rail, and across multiple islands, the cassowary.