Why is this important?
Because animals were likewise thought to be larger in the age of heroes as well.
However, the Chimera is more than just large.
You notice that the human giants in these pieces of art are proportioned like that of normal humans? That they are basically humans writ large?
Also, let's remember that the Chimera is a fire breather, and has three heads, one of them a poisonous snake. Indeed, its whole tail is a snake. Though Indominus Rex has the intelligence to counter the three-headed defenses of the Chimera, it doesn't have the speed to
Thus, I conclude...