I wonder what takes up that much space in Photoshop. I suspect the brush files, iirc those take up quite some space. I can imagine Pixelmator uses much more native / vertex images, like those you can create using PaintCode [1], which generates a bit of native Objective C code to produce the image you draw in that app. Paintcode is a great way to reduce the size of your app's downloaded content, btw.
