Hmm... on Windows, the "obstacles" are able to go out of bounds of the screen, but here it seems that your window manager is forcing the "obstacles" back into the screen, making the gap too small.
Sorry, but I'm not sure how else to help other than to suggest that you play it on Windows if you are able to. We've not had the chance to test this out properly on other OSs, so the experience of this game in any other OS is very hit or miss.