I'm probably going to add some more zoom options at some point. I think many would also prefer to have it the way your full screen works, Sebo, because many graphics cards default to stretching the image rather than showing black borders!
The thing about decimal zooms in a pixel based game is that the pixels gets distorted and everything appears stretched and/or blurred if they aren't multiplied by integers, so the game will look much worse than it should. Still, some players don't really care about that, and so it's on my to-do list for some future patch to add more advanced resolution options.