The surface electric field intensity $E$ is inversely proportional to the conductor radius $r$. By using bundle conductors, the equivalent radius $D_{eq}$ is significantly larger than a single conductor of the same cross-section. This reduction in $E$ keeps it below the disruptive critical voltage $V_c$, effectively suppressing corona discharge. Additionally, the spacing of sub-conductors increases the geometric mean distance (GMD) to radius ratio, which decreases the loop inductance $L = 2 \times 10^{-7} \ln(\frac{GMD}{GMR})$ and increases shunt capacitance $C = \frac{2\pi \epsilon}{\ln(\frac{GMD}{GMR})}$.

Remember that bundle conductors always modify the geometric mean radius (GMR) to be larger, which mathematically forces inductance down and capacitance up.