The article also provides us with this info: "The :executable t keyword argument includes the SBCL runtime in the image to ensure that the image is a standalon

The article also provides us with this info: "The :executable t keyword argument includes the SBCL runtime in the image to ensure that the image is a standalone executable. This is why the executable for even a simple hello-world program tends to be quite large (30 MB to 50 MB)!"