I loaded an 18 gig CSV file and the app used... 18 gig of RAM :)
There does seem to be a reference to 'out of core processing' here for how you might be able to address this with DuckDB:
https://duckdb.org/docs/guides/performance/how_to_tune_workloads.html#larger-than-memory-workloads-out-of-core-processing