Understood. But you have to consider that "good" is relative. Yes, she was beaten, but by some world champion fighters. This happens when you fight at such a high level. So I don't think we can say that her losing speed due to her muscular size would be the unequivocal reason for her losses. Sure there is a sacrifice of speed for size and power in boxing, but boxing like most sports isn't a simple equation...skills, decision making, toughness, desire, will, mistakes, etc. all play an often bigger role. As we often see in boxing, the physical matchup doesn't decide the outcome.
It could be (and I think it's far more likely) that those she fought great fighters and they were just better on that day. I think a better trained/prepared Jay Fuchs could be a world champion boxer WITH her current level of musculature. But she'd clearly need to make sure her endurance was sufficient as well, because that's also difficult to maintain with massive muscle size.
BTW: I was really referring to just sparring or training in boxing; but if she wanted to compete again that'd be cool too.
KJ