Hi Steve, I think you might have MDF and Plywood backwards as far as expansion goes but I'm not an expert. Look in this
thread for "Dave's_Not_Here"'s posts regarding sealing MDF and using Plywood. The sum (that I got) from his posts seems to indicate that properly sealed MDF won't change dimensionally whereas the only way to completely seal plywood is to wrap in plastic (Tyvex), otherwise it will change dimensions with the relative humidity changes. I hope this helps.