Hey Kayla!!!
I'm unsure of all of the possible causes -- I think there may be several, and I think that many of us probably deal with various combinations...
Some of us are on prednisone now and then -- and my understanding is that can cause "Cushing's Syndrome" -- among those symptoms can be belly fat ("muffin top", etc...), moon-pie face, thinning skin, bruising, insulin resistance... if we're not on prednisone now and then, or rather constantly, during our CF years -- we almost certainly are post lung transplant!
I also know a little pot-belly often seems associated with malnutrition...
In my years pre-transplant (I received two beautiful, new lungs almost 14 years ago...) my abs were pretty ripped -- I had VERY strong abdominal muscles from constantly coughing all of my life... In the past 12-13 years, that's been replaced by quite a bit of belly fat, which I do my bestest to keep at a reasonable level...
The bigger rib-cage is also very common for the reason you stated... For many of us -- it's called "barrel chested"... Many of our rib-cages look exaggerated compared to "normal" people -- some of us also get elongated lungs from our diaphragms pulling them down to suck in more air... Mine were abnormally long and I had to regularly guide/coach the x-ray techs so we didn't have to always re-take chest x-rays... (you know how that goes -- you try to tell them where your lungs are, they look at you like you're an idiot and take the pix they want to take, then apologize and re-take where you said your lungs were...)
Hope you're doing well!!! Love, Steve