PI x Radius squared equals area.
1 inch pipe = .7854 area.
2 inch pipe = 3.1416 area.
2 inch pipe is 4 times the area of the 1 inch pipe. The amount of water going through the pipe can vary with the pressure also.
Suggest asking a neighbor or two what their water bills average and then factor it out times the months and years involved.
In the event you are not satisfied with their adjustment then file suit in your local small claims court for the difference. Now you get a judge to hear the case and a lawyer is not required or acceptable in small claims court. The cost is very minor and will be refunded when you win the case.
Bear in mind that a water bill like any utility bill becomes a lien against the property and must be paid to clear the title to the property for transfer.
Bebop