This comparison is a little unfair. The 787-9 has to carry a lot of extra fuel to cover the extra range, and so it needs a stronger structure to carry the weight. This is one of the tradeoffs airlines and aircraft manufacturers have to deal with. A plane designed for a shorter range can shed a lot of the extra weight that would be needed to cover a longer range, making it more efficient for shorter flights.
Going by the same Wikipedia pages, the 787-9 has a MTOW of 254,011kg vs 204,116kg for the 767-400ER, and a range of 4000km further.