Rens Groenveld,firstly,I'd say sorry for the misunderstanding in my comment. After observing your screenshot, combining this blog, the partition key(in 2nd pic) is for logical partition and the partition name(in 3rd pic) is for physical partition.
You specify the partition key to create a logicalpartition that
guarantees to keep items with the same hash of the key together.
Cosmos DB manages the physical partitions based on needs. In the
portal, you can see that although we have a few dozen partition keys,
there are only a handful of partitions.
You could know the differences between logical partition and physical partition from this official document.
Unlike logical partitions, physical partitions are an internal
implementation of the system. You can't control their size, placement,
the count, or the mapping between the logical partitions and the
physical partitions. However, you can control the number of logical
partitions and the distribution of data and throughput by choosing the
right partition key.
Back to your issue, your logical partitioned data is keep together logically, cosmos db would balance them physically which we can't get involved. Data for the same logical partition maybe not reside on the same physical partition. I assumed that's the reason you remove the data does't save the size. I suggest you getting touch with Azure Cosmos DB Team to see what they can do with your physical partition.
