In fact, re-reading his full article, I am now wondering if my size rules go far enough. Basically, the stress put upon something is equal to its size difference. So if Zilla was simply ten times bigger than a t-rex (for the sake of this discussion) then it would need to be ten times stronger than its comparitive strength (by that I mean, even after we add strength bonuses for size differences it still needs to be ten times stronger than that...basically +17 strength if I remember my 3E.
But most of that strength just goes into supporting its body. It won't be that much stronger *functionally* - won't be able to *apply* that much strength.
Smaller things are stronger for their size. Ants can lift 50 times their own body weight - but they're actually pretty weak. A human scaled down to ant-size could lift *more* than that; an ant scaled up to human-size would be too weak to move, or close to it - it would need major redesign to survive at even cat-size, much less man-size.
Godzilla is going to be weaker
proportionallythan a T. rex.
Also, I'd say your Dire templates' HD increases (x2 / size category) are correct
for Dire-type creatures, which are supposed to be not just bigger but nastier/tougher, but not for generic size increases. A creature's HD shouldn't go up with size quite that easily. That's the equivalent of an increase in HD proportional to the increase in linear dimension (both double each size category). The MM/SRD porpoise is 4 to 6 feet long and has 2 HD. A whale 100 feet long, x20 times longer, shouldn't really have 40 HD - realistically, the biggest whales probably shouldn't have more than 15 HD, probably less; they really were fairly easy to kill. A 40 HD whale would make whaling nearly impossible.
For non-Dire size increasing, I'd say x1.75/size category, rounded up - so a Colossal whale comes out as 19 HD. Since the tail isn't supposed to be included in determining the size category, a Colossal whale's pretty huge, so that's acceptable.
With that rule...
a Colossal lion = 26-27 HD
a Gargantuan rhinoceros = 24-25 HD
a Colossal rhinoceros = 42-43 HD
a Gargantuan elephant = 19-20 HD
a Colossal elephant = 33-34 HD
a Titanic elephant = 58-59 HD
a Macro-Diminutive brown bear = 172-173 HD
...which all sound reasonable.
--- (Godzilla biomechanics stuff, ignore if you don't care about it) ---
The 500 tons thing just might be plausible, let's see. If one were to go straightforwardly from a 6 ton T-rex, a 500 ton T-rex would be cube root of (500/6) = 4.36 times larger in linear dimensions.
But AmeriGodzilla isn't built like a T-rex; a 60 meter tall T-rex would be about 150 meters long, and I don't think AmeriGodzilla is. It's probably more like 6 times larger in linear dimensions, adjusted for the different shape. (If you ignore the really tall back spikes, which are not going to weigh much, it's probably skinnier than a big T-rex would be, anyway.) So a 6x linear size T-rex will be 1296 tons. Reducing it to 500 tons gives AmeriGodzilla a density of 0.385 - insanely low, but maybe possible if AmeriGodzilla has a crazy air sac system (like a bird's) and highly pneumatized bones. (As far as I know, the lowest suggested for any real-world critter is a little above 0.5 - some pterosaurs may have been that air-filled.)
The interesting question is - could a 500 ton biped walk? I'm not sure.
This study's powerpoint (by an actual paleontologist!) suggests that a 10,000 ton Godzilla would not be able to support its weight, even standing.
But a 500 ton G. is a different matter. Using those methods, if 500 ton AmeriZilla had articular area of 4 square meters per leg (seems quite reasonable, those legs are
huge), the stress is actually less than on a big sauropod dinosaur. The trick is allometric scaling - the legs need to be ridiculously huge, bigger than AmeriZilla's even, in proportion to the body.
So, contra Mike Wong, I'm not willing to rule out a 500 ton biped as impossible in principle as a product of normal Earth biology. Dinosaurs didn't evolve one in reality - but given 100 million years or so of continual evolutionary pressure to get bigger, and near-infinite food supplies, the necessary biomechanical innovations are probably plausible.
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Erm, that was probably a bit tl;dr.
