I've been trying to shoot some examples but I don't have digital zoom on my camera and I'm starting to lose the light here. [Edit: losing the light also means that the sun is well over the yardarm and we have very high yardarms here]
OK, maybe try this.
You capture an image at 3000px x 2000px - total approx 6 Mb. You zoom in on that image at 2 x optical zoom. The image on the picture is twice as large but the pixels are still 3000 x 2000, approx 6 Mb the same. Now with a digital zoom you are still enlarging the image but reducing the pixels, i.e. 1500 x 1000. It's the same as taking an image and cropping it using a photo editor.
I'm not sure that makes sense as I'm not sure that I understand it myself!