If you register an image size of 1000×1000 then upload an image with an original resolution of 500×500, WordPress will resize it to create smaller versions, but it will not upscale the image to create larger versions.
This is because a larger version would be bigger but have the same resolution, WordPress can’t invent details like an AI so it will look pixelated and use up more disk space. Browsers are far better at displaying small images at larger sizes and can take into account the users hardware, though it will always be inferior to a higher resolution image.
If you upload smaller images and want to use larger image sizes, code needs to be written to test for this and fallback to a smaller size or the original image.
Alternatively, you can use upscaling tools and upload the larger upscaled version once you’re happy with the result, ensuring all image sizes are possible.
In the meantime, CSS can help in these situations with properties such as object-fit or background-cover to make the most of things and solve edge cases.