With many commercial magazine themes demanding large-scale images, the need for a Pixabay is readily apparent. So too its limitations. I searched for four terms -- forest, military, candy, and team -- all of which came back with impressive images, none of which were wider than 640px. Great for content within a post, but not for themes that use featured images in sliders.
Notably, the website itself offers the larger image sizes. Perhaps this is a glitch?