If this is a very literal question about what's it like being inside her, I'm pretty sure absolutely no difference.
The big difference is in the build up and anticipation, the fact that she looks so different to the average woman, the fact that when you touch her it's alll hard pulsing muscles rather than womanly curves and obviously the role reversal of being the weaker partner.
It's quite something to gaze across at a woman whose torso is more muscular than yours, her delts more pronounced and even her unflexed biceps bigger.
Unless she's quite gentle and yielding with you, the mismatch in power could mean the actuality of the sex isn't as pleasurable as the thought of it though. She could really batter you if she was on top and thrusting roughly and if you're on top you wouldn't be able to move if she decides to grip you tightly, so it will all be under her control.
Obviously most fbbs are not as strong as the average man and are nothing like this, but you have to assume that in questions like this about what fbbs and muscle women are like, the implied scenario is that we're talking about the really big strong girls, otherwise it's just a pointless musing about what fit women are like.