No overweight here and I have kept the same weight for the last 20 years while using MJ. All my "pot friends" are slim too.
During winter I am more lazy but it does not affect my weight because I naturally eat less.
In the summer, I cycle to work every day (25km round trip) and my weight will vary within 10 pounds. I usually loose about 5 pounds of fat and water at the beginning of the season, and then I gain it back in muscles, so overall I stay pretty much the same weight...
My personal perception is that MJ users in general are not overweight and I also notice that we usually look younger than we are. Or perhaps it's just me that likes to think I still look young