The ShareExtension allows the .hg folder of a checkout to share it's store with other checkouts, thus reducing the disk usage (cf. JENKINS-11363). However, this isn't applied (yet) for subrepositories.

So: for a simple repository "Repo" with one subrepository "Subrepo", i.e., 

• Repo 
  • Subrepo

 Checkouts of "Repo" use the sharing and does not contain it's own .hg/store.

 The recursive checkout of "Subrepo" does have it's own .hg/store, no data is shared.

Especially for large subrepositories, it would be nice if the .hg data could also be shared.