As they have said the Program download only contents the program and a minimum amount of content required to get Poser 13 up and running.  If you already have content from a prior version you don't have to download it again if you don't want.  however, they have updated a lot of the content to have Super Fly materials.  Anything that starts with P13,such as the Hivewire figures.  So if all you need is the program it is now about 448MB instead of the 1.6GB that poser 12 was.  

The one thing I like about the new program is how fast it is when rendering.  I can now get a decent preview render in about 15 seconds using a GTX1080 video card.  That card wouldn't even run OptiX under Poser 12 and now it is the fastest renderer for Poser 13.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13