BuddyPress.org

Then you just have to override that in your child theme. BP Default’s responsive.css https://buddypress.trac.wordpress.org/browser/tags/1.6.2/bp-themes/bp-default/_inc/css/responsive.css

See line 211 https://buddypress.trac.wordpress.org/browser/tags/1.6.2/bp-themes/bp-default/_inc/css/responsive.css#L210 where it expands #nav li to 100% of containing div. If you’re going to remove or change that, you’d need to work on for the #nav li which is set at 46% of container on line #43 of same file.