How did they make the 45mm 1.8 Olympus so small?

I disagree with both of these responses. As far as I'm aware, the 25mm is neither image stabilised nor a macro lens.

As for the apertures, the diameter of the 45mm aperture (45/1.8=25mm) is actually larger than the diameter of the 25mm aperture (25/1.4=17.9mm). So I too am amazed by the size difference.