Lurking and back-reading (I know, I know - for shame

)
Sea, saw your question - I have no definitive answer for you, though. The barrel chest associated with smoking is usually part of late-stage COPD (emphysema and the like). It occurs from air chronically being trapped in the lungs even during exhalation - thus always causing the ribs to stay more expanded as the lungs are always larger with the trapped air.
I do not know if at
that level of the disease if the process is reversible as the disease itself usually isn't.
I do know that people are different, though, and can't rule out that if one had some "barrel chesting" in early stages of COPD (not
usually the case) and the process was halted and lungs healed enough that as much air wasn't trapped as before that it wouldn't be possible to lose some chest size.
Weight loss and exercise changes, maybe Sea, since having quit? Those could be contributors to what you've noticed?
Just my