As soon as people caught up to BJJ they became irrelevant in MMA. Also when they faced somebody with ground game they had tons of issues (IE: Sakuraba "Gracie Killer")
Matter of fact recent performances by Gracies in MMA were embarrassing.
>http://www.mmamania.com/2010/2/10/1304038/cousin-renzo-rolle...
Etc.
It's directly related to testosterone level, but that's an artifact of training hard. Let's not claim it's all steroid use.
Pretty much, the article bears this out even.
'The researchers speculated that one causal mechanism may be higher testosterone levels - certainly past research has shown men with higher basal testosterone have wider faces, as well as greater strength and aggression.'
Though it's surely often misapplied, the terms 'gh/steroid/juice jaw' didn't develop out of thin air.
As an MMA coach, I think a more likely explanation is the wider faces correlate to high testosterone, including steroid use, which in turn correlates with more wins in the Octagon due to strength and stamina, not necessarily "fighting ability".
If you want to look at raw fighting ability, take a look at any member of the Gracie family, the best family of martial artists this last century and you will find they generally have skinnier faces.
http://www.bjjee.com/wp-content/uploads/2014/03/rener-ryron....