Even a comparatively small dragon can swallow a human-sized creature whole. Some hatch out of their eggs with jaws wide enough to take in a smallish human, albeit resulting an awkwardly full dragonet. As dragons grow human-sized meals eventually become trivial, but for a good while a single human is quite a good meal.

Since dragons can digest nearly anything and need metal in their diets to grow proper scales, armored humanoids are favored prey as they come with their own mineral supplements. Even good dragons will eat humans and humanoids if they judge them annoying enough. A lot of young dragons get killed due to this habit but that is small consolation to the many others who go to their reward knowing what it's like to be swallowed alive.

Smaller evil dragons, whose stomach will not hold a whole human, will often swallow people feetfirst so their prey will squirm longer. Having your legs digested while the dragon deliberately swallows air to keep you alive is not a good way to go. The lucky one go in headfirst.