Why does Guided Matching get Apex heap size errors?

Incoming records being processed by Guided Matching get stuck in Processing status. If the Status Reason field on the record has an Apex heap size too large error, you have hit a Salesforce Governor limit error in the org.

This error indicates you are processing too much data in Apex memory with the Guided Matching setup for that record. To avoid hitting the limit, go to the configuration of that Guided Matching setup and lower the Job Size setting.

Once you have reduced the job size, go back to the records with Processing status and retry Guided Matching. For Transaction records, for example, just click Retry unmatched Transactions on the Transaction Set Progress component to retry matching on all impacted records.