I apologize that you had to experience a bug like that.
Thank you for telling me about this.
I will look into the code and try to fix the error, and will fix this issue going forward.
For now, if you exit the game while fullscreen mode is off, next time you launch the game it should go back to the normal size.
Once again, my apologies you had to deal with that. I always try to bug test to the best of my own abilities, but sometimes I miss things. :)