I'm no authority on this at all, but on my 400 hrs savefile, the save time is about 15-17 seconds long. I built a decent bit around the world, but i noticed the time still increasing even when i was done building and just exploring the world in search for a Berserker trophy, and I don't think it's just the loot that makes the save file so big, so my honest assumption would be the discovered world to be saved along. Plus, a fresher world and character i'm currently playing (this time with very minimal building and terrain modification) are showing the same increase, really makes me think that's the main reason