A question I've seen debated again and again is over what, exactly, qualifies as a "giant" monster? Is it only Godzilla-sized monsters? Are Mighty Joe Young or Gwoemul from The Host (2006) too short? Should Japanese Daikaiju be considered separate from Western giant monsters? Where do dinosaurs or fantasy creatures stand?
I figured it was worth laying out my position on this issue.
In my opinion, if its abnormally large - usually, this means bigger than a truck - it's a giant monster. However, that doesn't mean every movie that features a giant monster should be considered or listed as a giant monster movie. Ghostbusters (1984) has the Stay Puft Marshmallow Man, but I doubt anyone really considers that a giant monster movie. Similarly, just because there's a large creature (or creatures!) running wild doesn't make it a giant monster movie either - I throw films like Jaws (1977), Jurassic Park (1993), and Primeval (2007) into this category, as they're more accurately classed as Killer Animal or Dinosaur pictures. I still enjoy these movies, though, and a lot of times I'll sort of arbitrarily declare this or that film to be a giant monster movie irregardless of any logic. ^_^()
Dinosaurs present a complicated case, because in my opinion they're sometimes giant monster movies, like The Valley of Gwangi (1968), but then sometimes they're not, like Jurassic Park.
So, if I boiled it down, what makes a giant monster movie a giant monster movie?
The assault on civilization.
One thing most giant monster movies share - from King Kong (1933) to Yonggary (1967) to Super 8 (2011) - is a conflict between the monster and society, often showcased by the monster attacking a city or being attacked by the Army (or both). The thing about regular Monster movies or Dinosaur movies is that, almost always, the monster's target is a person or group of people. In Jaws and The Beast (1996), the monster is more like the serial killer in a Slasher movie: picking people off, one by one, terrorizing a community or small isolated group of people. In Dinosaur movies, there are no targets at all - often, the story is about a group of characters trying to survive in a hostile environment (filled with man-eating beasts). In a Giant Monster movie, however, the monster's target is civilization itself. In Gojira (1954), The Beast from 20.000 Fathoms (1953), and Mothra (1961) the monsters sink ships and attack cities. There's no keeping their existence secret. In fact, there's more than one Giant Monster movie where the climax is the monster's reveal to the general public - most regular Monster movies never have that moment. Instead, the heroes spend the movie trying to convince the authorities that the Monster exists, often without success, and in many films the authorities' role is to show up at the end to claim the bodies. While the success rate of the police and military against giant monsters is, at best, unsatisfactory, they at least show up and get a chance to try.
I don't mean "Oh, a cop takes a shot but it does nothing." I mean, a Giant Monster movie will devote a significant portion of the film - a scene, a sequence - to the authority figures acknowledging the threat, devising a plan, and acting on it in an effort to defeat the monster. Hell, plenty of Giant Monster movies organize their plot around successive plans, each failing until the heroes finally devise one that works - Mothra vs. Godzilla (1964) or Gamera the Invincible (1965) are examples of this. This shifts the conflict of the film from "Hero v. Monster" to "Society v. Monster" - it's no longer about the hero, alone, struggling against the beast, instead it's now the hero and society itself, together, struggling to overcome the antagonist.
As I said, there are exceptions. I consider Anaconda (1997) a Giant Monster movie, even though it fits none of the criteria above. Why?
'Cause I like it, that's why. NYEH! :p