OEHLIN'R is the file containing the line details for historical Invoices and Credit Notes. This file is purged according to the options in the Company Profile | Optional/Specs tab (at the bottom). Check the Document Date on the Maintenance | Purge History function when you purge history.

ARTRAS'R is the Receivables transaction file. You need to purge history from there too. The transactions are purged either by billing cycle or period end. Date options are in the company profile.

DCHISD'R is the DataCare History log. Delete old history using the Hisory Maintenance menu in DataCare.

GLBTAD'R is the file of GL transaction batches. It will be purged at the end of the year.

BTW, given the price of disk space today, is there a reason that the file sizes are a problem?