BMI/body mass index relates to body composition, body fat to lean muscle mass, it has nothing to do with actual "size" in appearance. You have absolutely no idea what you are talking about, yet you continue to speak.
And if you really want to get technical, one pound of muscle being dense and compact in size takes up less space within the body than one pound of fat does. Visually consider what a 1 pound bar of lead would like compared to a 1 pound bag of cotton, and there you have it.
A 115 pound girl with 20% body fat will "appear" larger than a 115 pound girl with 10% body fat ... fact. Being more muscular does not equal being larger in size, and you are genuinely ignorant if you believe this.
You have based on your entire OP as it pertains to Penne on the fact that she used to fight at 105, which in your layman's brain means that she must be the physically smaller girl ... it does not.
In this case, Penne has a higher BMI than JJ, she carries more of her weight in fat, and therefore also carries more water = she has more water weight that she can lose than JJ.
Now if you had said "physically weaker" then your post might have some validity. More muscle does equal more strength, and the fact that Penne has more body fat and less muscle than JJ is Penne's fault. If she would train like an athlete she would develop more muscle mass. Go check out Penne's training vids, and then watch Joanna's .. night and day difference.
So this is not about "size" as you defined it, it is about strength. JJ earned hers, Penne can do the same. Next ...