How can I reduce the vertical spacing in a Google document's table of contents?

How can I reduce the vertical spacing in a Google document's table of contents?

For example, in one of my Google document, the vertical spacing in the table of contents used to be fine:

but after refreshing it, for some reason unknown to me, the vertical spacing has increased:

I would like to reduce the vertical spacing back to normal, i.e. as shown in the first screenshot.