I've had a play and the results agree with what I thought:
- Signing out of of Pocket and back in downloads all the articles in "My List".
- Signing out of Pocket and copying the configuration from another device, only downloaded those added since the last sync.
- Changing the "LastSync" to some time in the past downloaded all articles added since that date.
- The list of Archived articles was downloaded, but the articles were not.
I haven't done a lot of tests, but this agrees with what I have seen before.
I'm not sure how much that helps. Fiddling with the timestamp will reduce the articles downloaded. Whether that prunes them them to a suitable amount, I don't know.