Why am i losing weight but gaining muscle?

i thought muscles were heavier than fat! During summer, i work long hours in the sun.. hard physical work... 50 hours + a week... but come winter time, i do about 20-25 hours a week and its light work! i notice i get fatter during winter and lose some ammount of muscle! so at the end of every winter, im about 170 pounds... 5'11... but then after a few months of summer.. i drop down about 2 pants size... get really buffed , big arms , big muscle lol... also eat subway mcdo and harveys everyday during summer and i drop to 160 pounds! shouldnt i be heavier? and for the fastfood, my metabolism is still very good... no worry about that... in 11th and 12th grade, ate mcdonals every single day and didnt gain 1 pound! :P anyways if someone could tell me why im lighter when im more built up, it'd be appreciated! ty