please change videoQuality to videoBitrate.
This is @brion answer
Originally I think there was some concept that nobody would ever try to *stream* HD videos live, so they are 'for download' and use a fixed quality setting, which means they may have a much higher actual bitrate if there's a lot of high-frequency data or motion in the video, or a lower actual bitrate if they don't.
See also T70418.
My recommendation is to switch them to a fixed bitrate (or at least a constrained maximum bitrate) to make them friendlier to HD streaming.
Please see below on it being used.
WebVideoTranscode::ENC_WEBM_480P => array( 'maxSize' => '854x480', 'videoBitrate' => '1024', 'audioQuality' => '2', 'samplerate' => '44100', 'noUpscaling' => 'true', 'twopass' => 'true', 'keyframeInterval' => '128', 'bufDelay' => '256', 'videoCodec' => 'vp8', 'type' => 'video/webm; codecs="vp8, vorbis"', ),
and 720p looks like
WebVideoTranscode::ENC_WEBM_720P => array( 'maxSize' => '1280x720', 'videoQuality' => 7, 'audioQuality' => 3, 'noUpscaling' => 'true', 'videoCodec' => 'vp8', 'type' => 'video/webm; codecs="vp8, vorbis"', ),