It is kind of unrealistic, yes. Not everyone has the same body structure, this is true even when people aren't living in a post nuclear-war wasteland. Although this is one of those departures from reality that I can generally accept in games, just like the lack of any children (Even though the latter is one that Fallout doesn't have.) I understand that if the game needs to let characters wear different types of clothes and armor, making them have varied body types can be harder, so I generally just accept it. In fact, given the way that bodies are handled in the current engine, allowing characters to have different body shapes would likely require some round-about scripting, if you note, Oblivion had this too, in that case, it actually bothered me more because you had characters who aren't even human. In Fallout, you just have normal humans, ghouls, who I can accept as just normal humans retextured to look like rotting corpses, and super mutants who due to being considered "creatures" by the game mechanics already don't use the same models as normal people. Maybe if Bethesda actually decides to stop using this outdated and evidently horribly unstable engine in Fallout 4, assuming that's going to be the next Fallout game released, there might be a chance of seeing variation in body shape, but otherwise, short of mods, I don't see this changing much soon.
One could say that because it's post-apocolyptic America, that food is scarce and makes it difficult to be seen as noticably fat.
If that were the case, we'd probably see more people who are noticably underfed, yet we don't, everyone looks perfectly healthy. Also, while that explanation might work for people living in the wastes, it doesn't look to me like say... the people on the Strip are suffering from any lack of food.