When I teach, I use "big data" for data that won't fit in a single machine. "Small data" fits on a single machine in memory and medium data on disk.
Having said that duckDB is awesome. I recently ported a 20 year old Python app to modern Python. I made the backend swappable, polars or duckdb. Got a 40-80x speed improvement. Took 2 days.
I'm curious - what were you doing that polars was leaving a 40-80x speedup on the table? I've been happy with it's speed when held correctly, but it's certainly easy to hold it incorrectly and kill your perf if you're not careful