Is the thicker USB C cable included with the MacBook Pro charger necessary for its higher current capacity?
To answer my own question, according to Wikipedia, USB C cables can have different carrying capacities:


  All USB-C cables must be able to carry a minimum of 3 A current (at 20 V, 60 W) but can also carry high-power 5 A current (at 20 V, 100 W).[10] All USB-C to USB-C cables must contain e-marker chips programmed to identify the cable and its current capabilities. USB Charging ports should also be clearly marked with capable power wattage.[11]


So while I can't say this with 100% certainty, the cable that comes with the MBP charger probably is a higher capacity cable, but USB-C cables have chips in them to tell the device and charger their carrying capacity.