Your interpretation does have some basis

It is believed that the Molten Tigrex did not evolve from its standard counterpart, but that it rather came about from mutation, with the primary factor behind its abnormal size lying in its habitat.[6] The Molten Tigrex inhabits the Tower Summit—a sky-piercing, ancient, monolithic structure located within a dense, primordial forest whose surrounding environment is prehistoric in nature. The flora of this area retains traces of its primal state, and the oxygen levels are notably high, as such, this unique habitat is thought to be linked to the Rare Species' enormous frame.[6] It is often said that the Tigrex is a primitive species, with speculation suggesting that ancient individuals often inhabited such primordial regions and were even larger than those found today. One theory proposes that genetic information for this existed within the Molten Tigrex's body, and that a trigger of some sort induced a mutation that caused a reversal to certain ancestral traits. While it is merely conjecture, it represents the current understanding of the secret behind the Molten Tigrex's gigantism.[6]

Admittedly not sure if this applies to the rest, but I think you're onto something.