No question that it's an optical illusion. At that distance (at least several miles, presumably), a telephoto lens would "flatten" out the image, making depth perception nearly impossible.

Here's a few stats about the aircraft involved:

A300 - Length 177', wingspan 147'
777-300 - Length 242', wingspan 200'

The 777 is roughly 33% larger (wingspan and length), but the wingspan looks close to the same size as the Airbus in the picture (measured with Photoshop). The 777 is obviously longer in the picture, but the Airbus is turning away from the camera, making it appear shorter than it would if it were on the same heading as the 777. In my opinion, there's no way that the 777 looks 33% larger than the Airbus.

Just to test it out, I played with the image and resized the Airbus to make it 33% larger. With that change (which should have made the planes roughly the same size, if they were at the same distance), the Airbus looks significantly larger than the 777.

In short, it seems pretty clear that the Airbus was much closer to the camera than the 777, and given that there were no reports of a near miss from the Civil Aviation Authority or from either pilot, it's very unlikely that the planes were anywhere near as close as they appear in the photo. After all, there's no way that the Airbus pilot could have missed that 777 right in front of him!