I don't think we can assume PED use has bulked Cyborg up. The way someone trains is the primary means of bulking up, and PEDs allow the training that makes that possible. I don't think it's an advantage for an MMA fighter to add muscle bulk. I doubt it increases striking power. It adds strength, but if that means you have to fight in a bigger, stronger weight class, that benefit is lost. So I doubt Cyborg has trained or used PEDs to bulk up. Her preparations serve other goals.
Cyborg has never fought lighter than 145. She has failed to make weight at 145 before, and has even fought in a heavier weight class on one occasion. So coming down to 145 isn't even an easy cut for her. Making 135 would be a drastic cut that would threaten her power and stamina, with another "excuse fight" a likely result.
Remember when GSP said it would take him over a year to move up to middleweight for a fight with Anderson? That would be time training different and changing many things about his game. But a reigning champ doesn't have the luxury of interrupting their career that way. He never did it (unless that's what he's been doing lately . . . GSP-Weidman anyone?) Maybe Cyborg could over time rebuild herself to a lighter weight class and be effective, but I don't see her just booking a fight and doing it from where she is - it wouldn't be natural; it wouldn't be real.
I don't think these two will ever fight for the good reason that they aren't the same natural size. That doesn't bother me at all.