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Antagonist doesn't equate to villain though, because a villain can be a protagonist.
A dictionary definition is simply a matter of language mechanics. I agree with it but even in the "spirit" of the word Godzilla doesn't really qualify if you're not taking a biblical point of view, for instance, that can simply view the world with a two-dimensional assessment that anything not of God or praising Him is simply evil. I'm not saying you are, I'm just saying that's a POV I understand (without agreeing with it) because it needs no literary, logical or practical basis....a cruelly malicious person who is involved in or devoted to wickedness or crime; scoundrel; or a character in a play, novel, or the like, who constitutes an important evil agency in the plot.
Godzilla as villain isn't supported by his filmography, the intent of his creator, explicit exposition in the original film...I don't know what else to say, other than he's not fat either.
I wonder if that's coming from mostly Japanese folks or what. They have cultural issues with body aesthetics so I take those comments more or less with a grain of salt. The Toho guy in a suit was more dumpy looking, with no real physicality or even intuitive basis for his structure on real organisms. His shape was simply what could be done with a man in the suit and what you get as the result of these crude limitations on mobility and expression.
Artist renderings make him look better but this is where there seems to be a real issue with a weird kind of inverse dysmorphia where the idea of Godzilla is what people recall in their heads that tends to be better than the actual image of the suit. This latest design makes him look solid and muscular and more physically realistic without entirely losing either the classic, recognizable silhouette or a shape based on a theoretical guy in suit proportions.
XPlus-Godzilla-62-NightSky-Big.jpg
...that's frumpy, dumpy and fat, if we're going to start pointing fingers. The new one is built like Bane by comparison. Most of the classic Godzillas are actually People of Wal-Mart fucking fat.
XPlus-Godzilla-62-30s-Big.jpg
...he's pretty much always been a fat ass until now, sometimes slightly more pear shaped, sometimes mostly thunder-thighs, but I think ultimately these recent "fat" accusations are a case of "haters gonna hate" and regardless of anything else the simple fact that it's not a Japanese production will mean some irrational excuse will be found to fault the design or execution. The slack given to the campy, let's be real we're watching a full-body puppet show, Toho films is exclusive to those films and anything else gets, all of the sudden, folks looking without their Godzilla "beer goggles" and crying foul.