Hi,
Interesting, because those should be following the column width, I think you have found a bug there. I will make sure to correct it for the upcoming release.
Until then, this small custom CSS snippet should to the trick:
[html].asp_noui_lu {
min-width: 90%;
}
.noUi-target {
min-width: 90%;
}[/html]
Apply this code to your theme custom CSS field (if it supports it) or use the custom CSS field on the search plugin back-end.