Apex heap size too large error when merging Cases
During the Merge, this error is usually seen due to a large number of Email Messages on the Cases and the included large sized attachments.
Please enable the "Object Cloning Selection" option in the Setup Wizard:
Next, deselect "Email Messages" from the merge action being performed at that time:
To have it always deselected, under the Setup Wizard, deselect Email Messages from the Child Objects to be cloned: