Muscle mass is three times as heavy as fat mass...
Really "cut" or muscular guys can easy look lighter than they actually weigh... in fact, it's possible for a very muscular guy with little fat on him to be considered "obese" according to BMI definitions.
Yeah I think a lot of them do but it does depend on the build off the guy. According to BMI, I'm am half obese but I don't think I have any jiggly fat, all of it is toned and I guess most people would probably assume I weigh a lot less than I do. with other guys it could also be that they weigh less than they look. it really does depend on their build. :)
I'm 5'10", 310lbs, ~25% body fat, in my main profile pic (it's a recent pic). People I know tell me that I look like I weigh less. J_D_H is right on the money; muscle volume is 3x as dense as fat mass. Think about 1lb of feathers vs 1lb of iron. Same concept.