Previews and Masters in Photos Library
The contents of the Masters folder should be the basis for the Previews folder, I'm pretty sure the differences come from what computer or OS version the previews are generated in (retina vs non-retina making the most difference).

I think it only regenerates them when they already exist if you move a library to a retina computer from a non-retina computer using migration assistant or time machine (my Library was a little larger storage usage wise and thats what I put it down to at the time), and since El Capitan my non-retina Mac (a Mini) has been making bigger previews then it used to as well.

If all you really care about if making sure you have your originals, the Masters folder is all you need to keep backed up. In MUCH earlier versions of iPhoto, the Masters folder used to be called Originals, which made a tad more sense :P

All a little Anecdotal, I'm sorry, but I mess with iPhoto and now Photos libraries quite a bit. Just get bored ^_^