In case you didn’t, try the fullscreen button in the bottom right.
I agree it’s not a great default — the problem is that I did a pixel art design for 640 × 480, which means that the next scale up is 1280 × 960, which would be a bit uncomfortably large and put part of the game offscreen sometimes. If I do something like this again, I’d pick a smaller or larger base resolution with the itch.io page layout in mind.