Actually, Tubby is an acceptable nickname for your large friend as long as he's cool with it. so go ahead and call your fat buddy tubby, because as long aa the right people say it, it can be quite a good name.
A very fat person who loves to eat Burger King to the point that he believes he is the Burger King... He loves to live a Fat lifestyle and uses food to describe things and his favorite outing include food.
"Tubby must eat burger king on several occasions."