animate-mush replied to your post “I could probably explain why the dinosaur was so big in Deep Breath. …”
Yeah, I’m solidly in the “unattested Therapod” camp.
Well, it certainly is an unattested theropod, gargantuan size or not, but while tyrannosaurus rex is by no means the largest theropod out there the apparent size of this thing would need an overhaul of how physics applies to dinosaurs…
But fortunately we have the Silurians who are probably experts in genetic manipulation and could probably grow their own dinosaur to whatever size they liked; especially if they added structural enhancements.
And we know some Silurians left Earth altogether, so perhaps the Doctor picked it up from a space ship with low-gravity conditions, accounting for its size. Which means it probably was doing pretty well surviving on Earth for as long as it did, especially given the difference in oxygen levels between Victorian London and Cretaceous London. (Which I assume would have been the same on the spaceship as in the Cretaceous. Which actually leads to an interesting thought about Madame Vastra and her ability to “store oxygen” in her lungs.)
My last and slightly cheatier hypothesis is that when the TARDIS passed through the Cretaceous it materialised the wrong size, was swallowed up by the tyrannosaurid dinosaur, and when landing again it recalculated the dimensions of the outer shell to be the correct size during rematerialisation. But in doing so it rematerialised the dinosaur as proportionally the same size relative to itself during the Cretaceous and landed a 100+ metre tall dinosaur into the heart of 19th century London. Again, the dinosaur did well to survive for as long as it did, and would probably not have survived the journey back.