thanks for checking out the game!
so, it should display in fullscreen with the game area in the middle and black bars on either side. that's by design, and how it should work. hopefully it's what you're seeing.
if not, could you take a screenshot (press SHIFT + COMMAND + 3) and post it here so i could see what you're getting? my ability to test on Mac right now is limited but i'll do my best to help troubleshoot. thanks!