While it's ture that Kitsuchi has greater size of Doton, Onoki possess versatility. If Kitsuchi uses his mountain sandwich, Onoki simply makes it extremely heavy to the point where it can't move. He can overpower him in Taijutsu. He is faster, physically stronger and can apply his Doton: arm to CQC0. Lets not forget the golem that dwarfs a Bijuu.