Look at how Nunes was demolishing Pena in the first round and won it 10-9. Pena looked scared as hell. She dropped her twice and controlled the fight on the ground and also got get in a neck crank submission hold, which took a lot of her energy out. She tried to toy with Pena and paid a price for it. It was the same thing as the Zingano or Valentina 1 fight where Nunes destroyed both of them 10-8 in the first round, but they somehow survived the barrage of attacks and Nunes gassed and quit in the Zingano fight while she was able to manage to win the 2nd round against Valentina.
You could see in her speed, movement, hands dropped, on her face that she had left the building after round 1. Nina Nunes has been inflating her ego and she did not train seriously like she did for Cyborg, Holly, Miesha and Ronda. With their kid being their central focus, Pena's intel has been telling her, Nunes was barely in the gym until the final few weeks of the fight.
Nunes gassed and no matter how good one is, once you are gassed, you would look limp and sloppy because there is no energy for movement and power. At 135 lbs, Nunes always had an uphill battle in terms of cardio, if the fight did not finish early. At 145 lbs, she has cardio for days as seen in her Spencer fight.
Also, she has not been training very well and there is a theory that Nina has been domesticating a killer fighter who has known only how to fight since the age of 7.
That is why Nunes is still a big favorite for the rematch, but it is all going to depend on if she can fix her cardio issue and make her power, speed, movement consistent through the fight like she is able to at 145 lbs.