According to the Elders' manual "Shepherd the Flock of God", members CAN be disfellowshipped for gluttony and alcoholism. The catch? If you eat a bit of food or drink a sip of beer, that is considered "moderation". HOWEVER, if you take a single suck of smoke, that'll damn you.
In other words, general intake of food or alcohol is acceptable. According to the Elders's manual, gluttony is NOT to be judged by a person's size, so mere observation of weight means nothing. And intake of alcohol is okay if it isn't to excess.
There are no such parameters for tobacco: If you have ONE single "dose" of tobacco, you are ruined..
That is the difference.