I was playing the original Godzilla theme, then Atreyu's Quest from Neverending Story for the part with Rarity.
First of all, sorry for the long post.
Second, if you consider an Ursa Minor to be 30 feet when upright, then 300 feet for an U.Major is an absolute, off-the-charts exageration. The parent was, at most, 5 times larger than its cub...not 10.
And third, we can do al the comparitons and deductions we want, but in the end, the Ursa Minor was barely taller than a 2 story house when standing on its hind legs, while a clear shot of Greed Spike opening up a rooftop showed him to be at least 3 times taller than the same type of house.
That's all the compariton you need.
And yes, Ursa Majors are bigger than dragons (in the show) but not by as much as you make them out to be. They'd only be about 1.5 (2 times at most) times bigger, which is still not nearly enough to make up for the MASSIVE physical advantages that dragons are naturally born with.
Also, 1 meter = about 3 feet.
So 40 meters = about 120 feet.
Even by your calcs, that still makes these dragons 4 times bigger than an Ursa Minor.
Which puts them at about the same height as an Ursa Major.
About the gems: That's not how it works. Dragons are stated to eat gems. All gems so far, with no distinction. Every time a dragon eats gems, the show has NEVER shown, mentioned or even hinted that there's one specific gem they can't eat. And if the show doesn't do that, then you simply can't arbitrarily "decide" that there's one they can't eat to try and downplay the strength of their fangs.
Not only that, but the simple fact that Spike and all other dragons just grab and casually throw handfuls of gems (normally without even looking at them individually) into their mouths, should already be more than enough proof that they're not "worried" about there being a gem they can't chew on. So either you show any evidence that there IS one they can't eat, or we go by the show's canon. And that is, that dragons eat all gems without distinction.
About the lightning: It doesn't matter. Pegasus are not more resistant to damage than other ponies. They just have wings and can stand on clouds. That's it. The lightning jolts of lightning they produce are NOT the same as real world thunder. If we've seen ponies take them with hardly any damage, then trying to use them as an argument for an Ursa's dirability makes no sense. If anything, it shows the bear's hide is almost as resistant as a pony's, which actually makes sense as they take the same little lightnings the same way.
About Twilight's beams: Just like an Ursa of that size would tank middle-sized rocks (like the ones Twilight destroyed) no problem, it also can probably tank those beams as well. You're overestimating Twilight's beams there, since destroying a small rock wall is something even Iron Will could probably do. That wall was human/pony-sized. Against something the size of an Ursa? It would probably just annoy him. Or it could be Twilight knew that Ursa was a cub, and didn't want to hurt it. Whichever fits more. The point is, Twilight's beam are not anti-Kaijuu attacks (they're meant to hurt pony-sized targets) so don't treat them as such.
And we're comparing an Ursa's durability against a DRAGON's attacks, not a Pony's.
About Trixie's boasting: Because Ursas ARE a big deal for Ponies.
You have to understand that ponies, as a general species, are not capable of fighting off larger monsters like that. You have the Alicorn princesses, but you can count them with your hand, while the rest of the pony race is a different thing alltogether.
You go to a pony town boasting that you defeated an Ursa, you're gonna be treated as a hero because ponies, even in large groups, are normally helpless against a creature like that. If a pony goes and says she defeated a dragon though (and they believe her, which I doubt) she's gonna be made into a fucking legend, with parades, monuments and songs in her honor.
But none of that matters here, because we're talking about Dragons, not ponies.
About Dragon Fire:....Really?
You're gonna count the fire of a BABY dragon (smaller than even fillies) as an actual representation of the power of a dragon's fire, which can apparently singe the scales of a creature that swims around in lava no problem? C'mon dude. Get real.