Google+ has been adding more and more features ever since it was first open to registration. The latest feature to come to Google+ is the inclusion of ability to download photos shared on Google+ in full size. The technical lead Google+ photos has shared the details on this. The option to download the photos will be available for all photos shared unless the user has explicitly disabled this feature in his or her account.
To download photos in full size, open any shared photos on Google+ using the lightbox photo viewer and select Download photo option from the actions menu. In case you are sharing photos and do not want users to download it, you can disable the feature from the settings (https://plus.google.com/settings/plus
This feature was added based on the feedback from users.