First of all, you are right, that boar is nowhere comparable to an average mountain in size (at best this is barely small mountain lv) but at the end, the boar is still larger than the Dragon and I'll prove it:
As you can see, those fangs are at least 3x bigger than Zoro and considering that he only cut its head, Zoro's feat wasn't more impressive at all whereas we have this:
The houses weren't big at all? Do you see how tiny the people are compared to the buildings? The dragon's head was at least a bit bigger than those buildings, as you said, but at the end, those buildings effortlessly fit in that mouth, not to mention that the boar still looks ridiculously big in that smoke, several times larger than those buildings.
Yes, it is clearly not comparable to the Central Tree but it still easily dwarfs the other buildings utterly.
Like I said, we simply have to wait for the chapter since those drawings can be inconsistent as well, considering that the PH dragon is 70m which is actually very huge. Imo it is still not bigger or as big as the boar tho.