This.
Unity sounds a perfect engine when someone is planning a multiplatform product, but limits of inferior platforms that engine covers seem to choke the PC code. In other words, I bet the game saving/loading code in Unity is not optimized at all so we suffer from code based on phone/console limitations.
Another reminder, it's not only Unity that prolongs loading times based on existing savegames, the same thing plagues Sims 3 and it's not an Unity game. In Sims 3 the savegame gets bloated over time by keeping all sorts of irrelevant data, the solution is 3rd party saves cleaner tool, but that thing cannot help with Unity based products.