Your link to the screen grab does not work.
Looking in Chrome, it appears to be the
box-sizing:border-box; that is screwing it up.
Your "not" CSS looks incomplete. You would need to apply the not to the *:after and *:before too somehow.
An alternative would be to add to the circle player CSS the rule that returns the box-sizing to the default, whatever that is. Just make sure the rule is more specific that the *, which I doubt is hard.
http://www.w3schools.com/cssref/css3_pr_box-sizing.aspList all the circle player classes... I just listed 2:
.cp-container,
.cp-jplayer {
box-sizing:content-box;
}
If that is not specific enough... use div.cp-container... But It looks like you understand CSS so should know what I mean.