Thanks for the response, scrambler.
I don’t have that Jetpack feature enabled, and their Infinite Scroll feature is set to use the default theme behaviour.
It’s not just the image slider that’s small, it’s the search box above it, too. Both should be the full width of the page. It’s almost like there’s some random div that’s forcing it into columns on some pages, but not others. However, I can’t find anything like that in the coding when I use Inspect.