Part 1 was 2 GB and was causing performance issues so the dev decided to end the game and start a new game. The first release of Part 2 won't be big because their is very little content. As more content is added, Part 2 will get bigger. If the dev can't finish the story before Part 2 gets too big, we might get a Part 3.
Also the dev is taking the opportunity to add features like a chat and messaging system to the game, revamping the maps, and if my guess is right give the characters new FOG models. A lot of things to change so starting a new game is easier than updating existing one.