The memory limit setting is an internal setting designed to protect the indexer from running out of memory on very large files. It is based on the total RAM of the PC and some configuration settings in the config.xml.
IndexMemMgr.MaxRamUsageType
IndexMemMgr.MaxRamPerDocType
Values range from:
1 - Very High
2 - High
3 - Normal (default)
4 - Low
It's not possible to set these values using the UI, if you'd like more information please contact support:
https://www.mythicsoft.com/support/