Yeah I uh... saw a video ahem that made me question my understanding of female anatomy, and then, being autistic, began to work out the physics.

Yep, basically, even though the bad dragons have a core, that core is still ultimately just a more dense kind of a foam/sponge, more or less, than the rest of the... device. The whole thing is very capable of being squished into a smaller volume than it would otherwise assume in just... ambient air pressure.

Basically, the length gets transformed into width, doesn't even require the cervix to be penetrated, though it... is the kind of thing you'd probably need to legitimately train for.

As far as 'an actual monster pp would probably itself break before it could totally puncture the vaginal wall itself'...

... yup, ok, I checked. There's the EHS, but thats just a 4 stage psychometric model, doesn't actually correspond to ... actual material physics values. But there is also this...

https://pmc.ncbi.nlm.nih.gov/articles/PMC7475665/

... which actually does correspond to an actual physical value. For an EHS grade 4, essentially a raging erection, they got uh, ~225 +/- 25 kPa.

Given that human skin can resist roughly 690 kPa before being punctured, and a vaginal wall is... what, at least as, if not more elastic than skin?

Yeah your monster cock would need to be roughly 3x harder than a human cock seems to be caapble of being, for the scenario in the image to possibly play out.

I guess also for reference, 225 kPa is tougher than cartilidge, but less tough than cancellous bone, such as the ribs and shoulder bones, and also the flat skull bones.

So basically, you'd need to have a cock hard enough you could quite literally fuck someone's brains out with it, directly through the top of their head, before worrying about blasting through the vaginal wall becomes ... physically possible.