For a compression, the resoltuion of the picture is not changed, but the information is stored in a more efficient format.
Some image compressions work with omitting details, which cannot be seen by human (e.g. JPEG), others work lossless by exploiting similarities between the color values of neighboring pixels.
An example is a 200x300 image of just blue pixels. Then: "a 200x300 image of just blue pixels" is a compression already. Resizing is a completely different thing, e.g. when you create a 100x400 pixel image by interpolating.
