Jones was supposed to take him down and finish him with ease. It wasn't 'just' Gus' size. Jones manhandled guys similar to him in size or bigger before, O'Brien, Vera, and Bonnar.
Gus is always moving around, has good defensive grappling and TDD, and fast hands. When factor in the reach, that's what makes it even more difficult, meaning Jones can be hit where he couldn't be hit before.
Doesn't mean Jones can't gameplan for him. Gus has a couple of flaws that can easily be exploided. Jones exploited one of them with the head kicks and Gus lackadaisical approach to blocking them. Gus also turns his head and darts away when he's pressured. And when he's tired, he's not quick with it, so may be caught. Gus' lead leg can be kicked repeatedly to lesson his movement.
If Jones can cut Gus off, avoid too much damage, and work in some elbows and punches from the clinch, he'll have great success since Gus is weak there. Jones landed some good elbows inside during their fast fight and Cormier punished Gus inside with uppercuts.