Oh great! I guess the columns are not properly rendered because there are a lot of columns. That looks like a bug to me. I just created two issues and will release a bug fix soon.
I took a look at it appears the observable notebook has a stylesheet defined with a table element style that sets a table's max-width to 640px. That is why it is not expanding to fill the width. (I am not associated with the project)
Any idea why the columns aren't displaying correctly for wider tables? Take a look at this example:
https://observablehq.com/@simonw/grid-js-with-datasette?url=...