The source image is not a part of the project. Once you load it into your project, C2 makes a copy of that source image. Resizing it with the built-in image editor will update the image (in project, not the source) so also will affect the download size and memory usage.
If you would for instance resize the source file (in external editor), nothing would happen in the project itself.
Note that resizing the sprite object won't have such effect as this will resize only the object, not the image.