Local power companies will often feed a 200 amp service with #6 pole loop.
Power companies kind of do whatever they want. Our "new" building service feeders were a single 4/0 per phase for a service rated at 2800 amps/120-208v. I saw the install (1981) and questioned it. The electrician stated and I quote "From the meter to the vault it's Con-Ed's problem and they OK'd it" It took till 2011 or so before they fixed it and brought the service feeders up to the capacity of the SE.