Its size is mentioned by both Jenny and the Doctor (who calls it a giant dinosaur). Madame Vastra asserts that most dinosaurs were the same size and she should know as she was there.
This isn’t explored further during the episode but there is something unusual about this. Not only do we have real world evidence of dinosaurs true size (and Jenny mentions the fossils that had been found) but dinosaurs have also appeared in the show before such as ‘Invasion of the Dinosaurs’ and ‘Dinosaurs On A Spaceship’.
What could the explanation be for the difference in size?
Deciding on an answer is important as time travellers will likely encounter dinosaurs at some point.
VASTRA IS LYING
We only have Vastra’s word that other dinosaurs were a similar size. It could be that she doesn’t remember her past (aside from a few details) or never encountered a dinosaur (it could be that she never went into the wild).
It would not be out of character for her to lie to make herself appear more knowledgeable but she could be unaware that it isn’t true. The memory could have been implanted or she could believe that they were much bigger than they were because she was smaller at the time (suggesting she was only a small child when she witnessed a dinosaur).
VASTRA IS TELLING THE TRUTH AND FOSSILS ARE LIE
If dinosaurs were really this size where did the much smaller fossils come from? Could someone have created and planted them to hide the truth? Who would do such a thing and why?
A time travelling race like the Time Lords might have done it so that mankind would learn about the existence of dinosaurs without being terrified of them. This might be so their quest for knowledge isn’t stunted by fear or to prepare them for meeting other species.
Rather than being done for benevolent reasons another race might have obtained their own giant dinosaurs and preserved them. The fossils were planted so humanity knew what they were dealing with but were overwhelmed at the sight of a true dinosaur, once they were unleashed.
Now Vastra has revealed the truth about dinosaurs time travellers might want to investigate this cover up and find out who was responsible.
THIS IS A DIFFERENT RACE OF DINOSAURS
It could be that there is a whole breed of dinosaurs which were much larger versions of the species that we know. This breed is the one that Silurians are familiar with. To encounter other dinosaurs of similar size only requires a time traveller to arrive at the right period.
The question would remain whether the dinosaurs we know came before or after these giant versions. Did they shrink or did they grow to the size depicted here? If they shrunk it could suggest either their bodies became more efficient or they had to cope with diminished resources. If they grew it could indicate that their species was flourishing and could have could have grown larger if not wiped out.
THIS RACE OF DINOSAURS ISN’T NATIVE TO EARTH
We know that Mondas was a parallel to Earth and in Cybermen the Mondasians were sometimes described as being giants. It would stand to reason that parallel evolution would allow the existence of dinosaurs similar to our own just on a bigger scale.
This would indicate that Silurians also come from Mondas, rather than Earth. ‘Dinosaurs On A Spaceship’ revealed that the Silurians were capable of space travel. Could it be that they were moved off planet while in hibernation?
Awakening they only think this is their planet. The difference in size of the dinosaurs coudl tip Vastra off to the fact that this isn’t her world after all. If other Silurians learnt this it could completely change their stance that Earth is rightfully theirs.
THE TARDIS DID IT
Possibly the difference in size is due to the dinosaur swallowing the TARDIS. It could be that the time ship shrunk itself to prevent the dinosaur from choking (not wanting to accidentally kill the beast). When it re-materialised in Victorian London it returned to its normal size but in the process the dinosaur also grew larger.
This would indicate that the TARDIS can share its dimensional shifting abilities with beings clinging to its exterior. PCs might bear this in mind if their own TARDIS comes under attack by a monster. If they travel while it is still grappling the exterior they could make it a much bigger problem.
Had the dinosaur survived the process could possibly be reversed but would the Doctor be up to the challenge of piloting the time ship into its maw? Could they find a way to make the dinosaur swallow the TARDIS?
A similar distortion could have occurred due to the time travel technology in ‘Invasion of the Dinosaurs’. Similarly the Silurians might have shrunk the dinosaurs to make them easier to transport in ‘Dinosaurs In A Spaceship’.
THE UNIVERSE IS SHRINKING
The dinosaur is the same size as it has always been, it is the rest of the universe that is shrinking. It could be that the TARDIS adjusts its passengers to fit the scale of the rest of the universe where ever it travels but the dinosaur, on the exterior, retained its size.
If this were the case then this could be the start of a major crisis. Could the universe shrink down so small that would eventually vanish? Would it throw off the gravitational pull of entire galaxies?
Those travelling into the past without the aide of a TARDIS could find they are apparently shrinking the further into the past they travel. This might be what happened in ‘Planet of the Giants’ (which would indicate the universe is shrinking at an alarming rate).
Conversely travelling into the future would turn them into giants. Primitive time travellers could find that this compensates for finding themselves in the technologically advanced future. This could lead to a interesting scenario in which the PCs are perceived by locals as giant monsters and must find a way to resolve the situation before the military find them.
The Master’s tissue compressor could actually be a device that accelerates the process, rapidly turning victims into doll sized versions of themselves. This wouldn’t be too dissimilar to what he does to the Doctor in ‘The Sound of Drums’ where the Doctor is not only aged but shrunk.