Version 2.1 changed the archive format to .tar.
Tar files are not compressed, so the size of a .tar archived project is about 10 times an archive produced with earlier version of the plugin.
Install shelve-project-plugin version < 2.1 and shelve a project.
Take note of the .zip file size
Install version 2.1, unshelve the project, then shelve it again.
Compare the size of the .tar file to the size of the previous .zip file.
Suggest to compress the .tar file into a .tar.gz instead.