We bill based on size of disconnect rating not the fuse size.
bill for what?
Most POCO here have construction charges to initially provide service but is based on actual equipment they provide, and may even consider the estimated load and return on investment in energy sales revenues.
I have recently put in 400 amp 480 volt service gear at a grain storage site, but the load could have been handled by 150 amp or less. POCO only provided transformer capacity for the actual connected load which was about 90 Hp of motors expected to run simultaneously at any one time and not the 400 amp capacity of my service disconnecting means. I think they only provided 75KVA transformer bank, maybe 112.5. If customer increases the load someday they will change the transformers and bill accordingly/credit for the old ones.
Same customer had another site that we added more load to this past season, same POCO changed a 300 KVA transformer to a 500 - I never changed any of the service equipment though as it had been sized for future expansion.