In addition to the fairly high resolution, if you look at the image at 100% you'll see that it's incredibly (excessively, if you ask me) grainy, probably from film grain simulation, which is all the rage among some photographers right now. Simulated film grain is more or less spraying a random number generator across your picture, and it does not compress well because (pseudo) random values don't have patterns to take advantage of. So a large portion of the file size is taken up by recording all that digital noise you see when you zoom in.
