Holly Holm: Cyborg's a Tougher Fight Than Rousey, Here's Why

Holly Holm shocked the world when she slept Ronda Rousey back at UFC 193 ... but Holly says Cris Cyborg -- not Rousey -- is 100% "the biggest challenge" of her career.

Scary thing is ... she's probably right.

"I'm going up in weight and taking on not just a heavier opponent in a bigger weight class, but also a very dominant one," Holm told the guys on "TMZ Live."

She ain't kidding -- Cyborg hasn't lost in over a decade, and KO'd almost every chick brave enough to step into the cage with her.

But Holly ain't backin' down -- "That doesn't mean that she's invincible -- she’s still human-ish."

Bonus: Since she's moving up for the Dec. 30 scrap, we had to ask Holm if she's cool to get her grub on over the holiday weekend.

Short answer -- hell no!