This isn't true. There are already large dinosaurs in the Triassic, especially the basal sauropodomorphs, also known as "prosauropods", some of which can grow up to multiple tonnes in mass.
But there is one.
One basal sauropodomorph to rule them all.
One so large, it would completely change your entire concept of Triassic animal life.
One so large, that it takes the throne of the largest known biped.
Unfortunately, it doesn't have a name yet.
This paper: A Diplodocus-sized basal sauropodomorph from the Late Triassic of South Africa
This piece of scientific literature speaks of a dinosaur in the legendary size class by pre-Jurassic standards. Estimated at up to ~15 tonnes, it competes in size with midsize Jurassic sauropods such as Diplodocus, and throws Spinosaurus right off the throne of largest known biped.
It also shows that you don't need to be a Jurassic or a Cretaceous dinosaur to be a legendary giant.
This dinosaur is closely related to Aardonyx. It's bones are, on average, ~190% of the dimensions of Aardonyx bones. It's femur alone is ~1.5 meters long, longer than any known theropod femur.
This image below should give you an idea of size. The king of the tyrant dinosaurs only just about matches the enormous Triassic legend in height only, and consider that basal sauropodomorphs are pretty low-slung for their size. This comparison doesn't even address the increase in bulkiness and robusticity that the large size would have required due to natural scaling laws.
|Maybe we should reconsider about how we picture the Triassic period. Maybe it wasn't too different from the two later periods.|
Tyrannosaurus based on Scott Hartman's skeletal.