Thanks for your explanation, makes sense to me :)
I tried your suggestion of bumping the `cache_size` in my test script and your theory checks out.
For anyone interested:
For the single big incremental vacuum:
Page Count: 24482 Freelist Count: 24206
Starting one big incremental vacuum
Total pages written to WAL: 247 Total pages moved to DB: 247
Page Count: 247 Freelist Count: 0
And for repeated small vacuums:
Page Count: 24482 Freelist Count: 24206
Starting 122 calls to incremental_vacuum with 200 pages
Total pages written to WAL: 544 Total pages moved to DB: 544
Page Count: 247 Freelist Count: 0