And how do you suggest they got so big in such a short amount of time?
Imagine that question, but this time imagine it with an all-powerful God in mind.
Well, then you are talking about Noah's story being magic--that is, not possible given the basics of science. That is changing the subject. We are explaining why the Noah story cannot be true according to science. Science is about things that are possible given physics, biology, climatology, geology, astronomy, archaeology, etc.
Magic does not have to take any of that into account. We all agree that the Noah story has to be a fairy tale based on magic.
But we were discussing what is scientifically possible
. Not what is magically possible if all bets are off, powerful but undetectable fairies are real, anything can happen, and you can't trust the evidence of your senses or any measuring instruments. And all the evidence magically disappears.
You are proposing that dinosaurs and other species quickly evolved into different sorts of animals right in front of people as if by time lapse photography. And it had never happened before or since. Because of god magic. Each generation of lizard got gigantically bigger and bigger, eating entire forests of food and then one day they all disappeared. But nobody noticed it happening or recorded it in any way. No ancient Chinese or Egyptian records of anyone eating dinosaur or being eaten by them.
No fossil dino bones found in the ruins of any ancient civilizations. No Jurassic park scenarios anywhere.
Because that world where anything can happen but not in front of anybody and without never leaving any physical evidence that it happened? That is not the world we have. Sorry.