Monster Hunter takes place in a low gravity worldHunters seem to be capable of falling from great hights without getting hurt, animals can grow to immense sizes, especially invertebrates whose exoeskeleton can hold their weight with easy, compared to their Earth counterparts. The low gravity could also make it possible for large creatures to move under sand and why weapons are so huge, the hunters can carry them because they feel lighter.
Odd volcanic behaviorThe volcanic areas present on the game seem to be strangely static, showing little change despite it's very active magma flows. Said magma flows work more like rivers (and waterfalls). The minerals present in these areas seem to be able to stay solid even at very high temperatures, that explains why some areas remain unchanged, and the native creatures in these areas, by absorbing said minerals these creatures are able to resist the ridiculously high temperatures and some even dive in magma without problems
Hunters are not humanThe Hunters may look human but they are in fact some form of super human. They can carry and effectively use weapons as large if not larger then themselves, they can survive forces that would reduce a human to a smear on the ground, and they are capable of holding their breath for extreme lengths of time and draw oxygen from oxygenated water (passing through small patches of bubbles refill your breath meter)
The monsters are the end result of Gaia's Vengeance and the Hunters were created by humanity.Think about it: when a monster takes you down, they don't eat you, they leave your body, thinking you're dead. That can only mean that the monsters are intelligent enough to know that humans are the most dangerous creature on the planet. Ignoring the gameplay mechanics and looking at the world itself, the monsters may have been the result of evolution, like humans kept killing and killing and only bigger and nastier animals were left behind. The more humans kept invading their territories, the more humans kept dying. The end result? Monsters that (allegedly) eat humans, thus explaining why they're so big: humans are fucking EVERYWHERE, so with enough humans, monsters would have tons of nutrition to grow big. Hunters were created to combat the threat.
The Ceadeus is based off of the Sumerian god EnkiThe Ceadeus has traits of both a goat (The prominent horns and "beard") and a fish (living in the water and having a whale-like tail), both sacred animals of Enki. (Which were later combined to form the "Capricorn", which again the Ceadeus has an uncanny resemblance to) As well, both Enki and the spoiler: Ceadeus live in underground oceans. (Though freshwater in the case of Enki and saltwater in the case of the spoiler: Ceadeus) Also, the Akkadian version of Enki's name is "Ea", which is right there in the name: cEAdeus. Pacific Rim, the Kaiju invaded the Monster Hunter world through a rift in the ocean floor. However, as explained in the movie, a pristine, earth-like world is not ideal for Kaiju colonization, so the rift was closed and the remaining Kaiju were left to fend for themselves. While some of the Kaiju's traits, more practical for indiscriminate killing than survival over a long period of time, were evolved out, such as their massive size, Multi-Armed and Dangerous traits and Bloody Murder abilities, some of their impractical traits, such as highly specific and energy-consuming weaponry and needless aggression remained. The result, of course, are the titular monsters of the game, which highly resemble the Kaiju. The Pseudo-Wyverns, who have wings that double as arms and run on all fours like cats, and the Cave Wyverns (The Gigginox and Khezu), who have similar attributes to Pseudo-Wyverns, use poisonous sacs and have prehensile tails, are descended from Otachi clones, while Brute Wyverns, who are dinosaur-like in behaviour and size and tend to have distinctive crests on their head that can be used in a charge attack, are descended from the clones of the Trespasser/Knifehead/Scunner-type Kaiju. Leviathans, who have crocodilian body structure and are the most adapted to underwater combat, come from Raiju clones. Elder Dragons are descended from the enormous, mysterious, Class-5 Kaiju.