During initial setup you gave Arq a budget for S3 storage. To edit the budget, open "Preferences" (from the "Arq" menu) and select the "Budget" tab.
You can enter the budget number in either dollars/month or GB.
Arq periodically calculates the total storage being used in S3 for your computer's backups. If the total is over the budget, Arq drops the oldest backups across all backed-up folders until the storage is within the budget.
NOTE: Arq always keeps at least 1 backup of each folder, even if that means the storage is over budget.