According to Dr. AnnMaria De Mars, Rousey's mother, the answer is not complicated: The dominant Brazilian fighter is using performance-enhancing drugs.
She put the former Strikeforce featherweight champ on blast during the latest edition of Ariel Helwani's The MMA Hour.
"We all know she's doing steroids," De Mars said on the show. "It's like the guy who gets caught with a DUI and says "That's the first time I've ever drove drunk in my life." Well, it's possible, but its not too likely."
Santos served a year-long suspension from December 2011 until December 2012 for testing positive for the anabolic steroid stanozolol after a knockout win over Hiroko Yamanaka.
While the lean, muscular Santos denied any wrongdoing, blaming the positive test on an over-the-counter supplement, many fans and analysts, as well as the California State Athletic Commission, dismissed that notion.
De Mars was also anything but sympathetic about Cyborg's claims that a cut down to bantamweight, the UFC's only weight class for women, would be detrimental to her long-term health.
"I would say [if] you're a disciplined athlete and you can be disciplined enough to monitor your training and your weight and make your weight you committed to, why can't she do that, too? Why should she get special privileges? I don't believe in special rules for special people, and I think all of her reasons for not making weight are false ... I say that because I competed for many, many years, and the thing about you can't cut weight because it might damage your chance of having children? Puh-leeze. I had four kids and I competed for 14 years and I cut all the way down. To make weight once for three minutes to stand on a scale and someone offers you a bucket of money, and you say you can't do it?"
Back in November, Cyborg's camp claimed her personal doctor forbade a cut down to 135 pounds, as it could prevent her from having children later in life.
De Mars is no stranger to cutting weight for competition, as she is a long-time Judo practitioner who won a gold medal in the sport at the 1984 World Judo competition.
While Rousey's mother obviously has a biased opinion in the matter, how realistic is her evaluation of Santos' recent choices regarding her mixed martial arts career?