If the csa of 107 mm2 is correct, then a solid round bar of that area has a diameter of 11.672 mm. That is without any insulation or protective sheath. A stranded conductor will be slightly larger than this owing to small interstices between the strands. That probably accounts for the size you cite.
I strongly advise that you do not remove the insulation from a main incomer in order to fit a current transformer.