Main service panel wiring
Code requires a minimum 12 AWG wire for a 20-amp breaker. You are fine with #12 wire. I have wired quite a few homes where the owners wanted 10-gauge wiring. 10-gauge can carry more current but is not needed unless it is a very long run: over 100 feet.  Just to give an idea, #12 is rated for 40 amps with THHN but residential is limited to 20 amps for safety.