I smoke cannabis and drink red wine am not overweight. Don't really exercise. Cannabis definitely helps alleviate built up stress.
I smoke and drink beer, but the difference is I drink fewer beers when stoned than I typically do when sober. I'll probably drink three or four depending on the day, but if I'm stoned, I might drink one beer? Maybe two? Typically though it's zero.
While that seems like an obvious factor, I'm sure there are other factors too.
For instance, people getting high on cannabis probably won't drink as much beer or other forms of alcohol. This probably contributes to a generally lower weight.