Larger models better understand and reproduce what's in their training set.

For example, I used to get verbatim quotes and answers from copyrighted works when I used GPT-3.5. That's what clued me in to the copyright problem. Whereas, the smallest models often produced nonsense about the same topics. Because small models often produce nonsense.

You might need to do a new test each time to avoid your old ones being scraped into the training sets. Maybe a new one for each model produced after your last one. Totally unrelated to the last one, too.