Ronda Rousey's too chicken to fight Cris 'Cyborg' Santos ... and that's why Cyborg won't be facing Rousey in Rousey's first ever UFC fight next February -- Cyborg's rep tells TMZ.
FYI -- Cyborg is widely regarded as one of the most dominant female MMA fighters on the planet ... famously beating up former "American Gladiators" star Gina Carano back in 2009.
According to the rep, "This is a fight all the fans want and we want to give to them. We have offered to fight Ronda at a catch weight of 140lbs. She has turned that offer down and will not budge to make this fight happen."
The rep adds, "Cyborg wants Rousey but can not and will not drop to 135 [Rousey's fighting weight]."
According to the rep, "We understand Ronda is worried about losing because of her new found fame. We are not worried about fame; we are worried about Cris' health."
The rep tells us, several doctors have said it would be unsafe for Cris (who fights at 145) to drop ten pounds for a bout.
TMZ broke the story ... Rousey will face off against Liz Carmouche at UFC 157 instead.