Reducing Pocket app size
Pocket's customer support gave this information to another user who had a very similar problem (albeit in 2014):


  [When an item has] been Archived or Deleted ... we attempt to remove all associated files. However, we have learned more recently that some information can be retained under certain circumstances. This is why logging out and back in dropped Pocket’s storage usage so dramatically.


Logging out and back in causes the entire local storage to be cleared, at which point only items on your List (not Archived) are re-downloaded.

They also said they were adding a way to limit storage usage as well as make cache cleanup more efficient, though that does not seem to be true in your case.