Don't sweat it. Unless you have a car, you'll be walking everywhere in Japan (great way to see the sights) and with the smaller/healthier food portions, you'll drop the weight pretty easily. I spent a year teaching English and easily went from a slightly muscular 200lbs to 180lbs (still muscular by Japanese standards). In fact, most of the guys I worked with lost weight, but strangely most of the girls gained weight. At 6'0", unless you drop down to 160, or so, you'll still be considered big in Japan. Some of the young guys are thinner than their girlfriends