Compare two large indexes with transform preview

Hi, I'm pretty new to Elastic so please bear with me. I'm trying to compare two large indexes (1.7 million and 400k) for regression testing purposes, which should contain exactly the same data once the larger index is filtered.
As a PoC I've tried it with 2 small indexes using POST _transform/_preview
This worked fine and I have an API that gets the results of this preview and prints a sort of summary. Now I am trying to replicate the same thing for the larger indexes but it seems like only the first 100 rows are compared/considered. I've been reading the documentation but I'm very confused about how I would go about achieving the same result for the large indexes.

Thanks in advance