You can go into Resource Monitor and look at Memory usage. There’s an item for ZFS ARC but I am not sure if that is the memory used for De-duplication.
It may be helpful to see how much memory you are using and you can change the amount of memory that ARC uses in Storage and Snapshots. What level is set for yours? Can you increase it?
As you increase your size, I would expect the de-duplication could decrease because how much of that new content of data is duplicate data?