Currently we use a fixed bitrate for webm (and ogg for that matter) transcodes except for webm 720p where we use a variable bit rate( -qmin 18 -qmax 18).
I just noticed there are some files, such as [[File:Hindesite_-_Lily_-_Focus_Stacking_(by).ogv]], where the 720p webm transcode is smaller (2733005 bytes) than the 360p webm transcode (2769705 bytes). This seems wrong. (Note, this is probably specific to the lily file which has a lot of still frames)
I don't know what original considerations went into using a fixed bitrate, but maybe we should just be specifying a quality level instead.
In my test of the lily file, using a quality level of 18 (like the current 720p setting) for the 480p version gave a file size of 1.5mb, compared to the current 480p settings of -qmin 1 -qmax 51 -vb '1024000' which gives a file size of 5.3mb.