Right. If we're going to dismiss weapon effectiveness based on physical realities of mass and strength, we similarly should be accounting for how those same physical realities alter and limit the large monsters...
...Those are still huge, of course, and as SP aptly pointed out, a hippo is enormously, terrifyingly more powerful than a human, at a "mere" 10-16 ft in length. Though dragons are at least somewhat serpentine, with long tails and necks which add to the length, and if we are assuming flight we could also reasonably assume a lighter build is part of that. A dragon 30' or even 50' in total length might be considerably less massive than a 15' hippo.
And in addition to the size, fantasy isn't always monolithic in how mythic or heroically it treats dragons and dragonslaying. In Barbara Hambly's 1985 Dragonsbane, our protagonist hero is a bookish local lord who's earned that eponym by cornering a smallish local dragon in a cave and killing it with a poisoned spear/harpoon. Inflicting a telling wound and getting TFO, as I recall.
This is all to say that while the basic premise of a much smaller animal fighting and killing a much larger animal has multiple basic ways for it to be doubtful and implausible, there are also ways of addressing and adjusting the fiction to make it MORE plausible, if we so choose.