Another factor not yet explicitly mentioned often plays a crucial role when sizing battery cables for UPS systems. Namely you want to factor in how the voltage drop from the resistance of the cables impacts the maximum runtime of the UPS. The higher the resistance of the cables, the more voltage drop they cause under load, hence the quicker termination voltage is reached (UPS shutdown due to undervoltage protection). If the wrong choice is made then the load may not have enough time to properly shutdown (or the usable life of the batteries may be too short). This is especially important for high-current loads since the voltage drop is proportional to the current \$ V = I\cdot R \$ by Ohm's law.
Choosing the shorter and largest diameter cables minimizes voltage losses due to cable resistance so increases the UPS runtime. It will also increase the usable lifetime of the batteries.
Many UPS manufacturers include formulas and tables that aid in sizing cables and calculating runtime estimates. You may find those very helpful (even if they are for different but similar UPS systems).