Its because you cannot see the fuselage! It is much bigger in comparison which makes the engines look smaller. Your eyes are comparing the engine with the people, and thus it looks exceptionally large. Im sure if you have the same angle with a GE90, it will look even bigger.
The engines on the 340-300 are not very large at all. The engine in the picture is far in the forefront versus the people. The photo taker used a small apeture to keep everything in focus. It's just a optical illusion.