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UFC star Julianna Pena just RIPPED into Ronda Rousey -- saying, "No one cares about [her] and her fat arms anymore."

"She's like the emperor in no clothes," Pena added ... "She's broken a little bit."

So, what set Pena off?

We asked her about Amanda Nunes saying she would gladly fight her ... if she couldn't book Ronda. And while Pena says she understands from a business perspective, she thinks Ronda's days as the top dog in UFC are over.

As for Ronda's long-awaited return, Pena says she's heard rumblings that RR will fight again before the end of the year ... but adds, "I'll believe it when I see it."

Until then, Pena says it's only a matter of time until she crushes Nunes and takes the belt.

"They can delay me but they can't deny me."