Why are automotive connectors and wires so big?
High heat, weather and vibration environment. Lower gauge, high strand count wire is used to reduce resistance, increase physical strength and longevity. Large connectors are used to ensure they stay together and these connectors are often waterproof and high heat tolerances. The cable insulator and connector plastic is oil, gas and other automotive fluid resistant as well.

If multibillion dollar companies thought they could shave pennies off every cable and connector by using smaller ones, they would.