McMurphy can be seen as "bigger" than the Chief in a couple of ways. The first is that McMurphy is the reason why the Chief ends up embracing change. McMurphy is the ultimate non- conformist, the sole voice of dissent against Nurse Ratched's extreme sense of control. In this, he can be seen as "bigger" than anyone else because he fights her at every juncture to represent the force of freedom over oppressive control.
This becomes another reason why McMurphy can be seen as "bigger" than the Chief. Nurse Ratched recognizes McMurphy as the primary threat to her sense of controlling design. Even the doctors in the ward hold little power in comparison to her. Nurse Ratched is legitimately concerned that McMurphy can initiate change. In this, she perceives him to be her only formidable threat. It is in this light where he is seen as "bigger" than the Chief.
Certainly, the ending in which the Chief has to kill McMurphy might be representative of how McMurphy is seen. The Chief cannot stand to see the sight of the lobotomized McMurphy. At this moment, the legacy of McMurphy is larger than the physical condition of either McMurphy or the Chief. It is for this reason why the Chief suffocates him. The Chief recognizes that there is something larger at stake. This presence dwarfs both the lobotomized McMurphy or the Chief, a presence that Mac himself had embodied throughout the narrative. In this understanding, McMurphy is "bigger," in a sense, than the Chief.