BuddyPress.org

You could try redefining $content_width if a mobile device is on your site.

Maybe something like:

if ( wp_is_mobile() ) {
    $GLOBALS['content_width'] = 100; // change 100 to whatever your value should be. this shouldn't be a percentage value.
}

This is probably too generic though.

danbp’s idea of using CSS media queries is probably better. Or consider using a JS script like fluidvids to accomplish what you need.