I don't know a lot about the production history of The Expanse. I know it moved from SyFy to Amazon with Season 4, and that the 6th season was only 6 episodes (for comparison, Season 2 contained 13 episodes). Further, the number of sets and scope of those sets are greatly reduced when you think of the grandeur of the first seasons, with multiple space stations, practical locations on Earth, interiors of a variety of ships, etc... heck, casting someone like Jared Harris as a supporting role was nothing to sneeze at. And we always saw an army of extras.
That said, this season there's at least half an army of extras, the limited sets we do see are as detailed as ever (if no longer multi-story and as deeply layered), and the VFX are still rock solid.
Based on the 6th book of the series and a novella, Season 6 is the most direct continuation from one season to the next - picking up as our crew, who dispersed across the solar system in Season 5, reunited on the Roci in the wake of Alex's death. Now, they're out hunting as part of the UNN/ UMRC coalition, with Peaches/ Clarissa on board. Avasarala is leading the UN and working with Martian leadership, while Camina Drummer is out in the belt resisting Marco in the wake of the death of Ashford and Fred Johnson.