The 747 actually had rather small engines. First of all, it had 4 and they were older-style with low bypass ratio. The 777 isn't that far from 747 capacity, but

The 747 actually had rather small engines. First of all, it had 4 and they were older-style with low bypass ratio. The 777 isn't that far from 747 capacity, but only sports 2 gigantic engines, they not only need to be more powerful, but have high bypass ratios to improve efficiency. They are even wider than the body of a 737.