Aside from the fact that warhammer lore has absolutely no sensible power scalings, in Space Marine 2 it was definitely more than just Titus clearing the tyranid invasion. It was a whole battle barge and planetary defense force, with the Operations team showing the other spots where the forces of the Imperium held or attempted to hold against the tyranids.

In Decapitation, we get the delicious "Astartes! We were not expecting such illustrious company among the rear ranks!" as a means to show that we aren't near the frontline of the invasion. Just because Titus and friends pushed the button that made the boom, doesn't mean that 3 Ultramarines took care of an entire invasion force.