There was this episode on Maury a few years back on couples who were completely opposite physically. There were couples who completely differ in height, weight, race, and physical features including occupations. One guest had the hugest breasts in the world, (!) and her husband worked at a typical office type of job, which meant he usually carries a suitcase around. As the shock is settling after seeing the woman, when you see the thinning hair on this guy, you wonder, how in the world did he get HER??!! It so happens that they met at a bar(doesn't it usually happen this way?), so you can imagine all the pervs whistling at her as she put on her show, so how did THIS guy get in the picture? Reality check, its surprising too: when she noticed him, they locked eyes. She said that he fell for her eyes, it is what he loves most about her. When he talks to her or looks at her, the first thing he looks at are her eyes, not her huge puppies. The huge breasts are just a plus but not why he wanted her. So what I'm trying to say is that they would not be together just because they are physically opposite, no matter how much of a plus it is.
Opposites tend to compliment each other, perhaps the weight may be the good difference. Every other important element needed would be there, just as how it should be for any and every "normal" relationship, if you want to put it that way. The relationship between them wouldn't be, well we're together cuz she's fat and I'm skinny. It is just because they care about each other, it's not a shocker.
It is definitely not impossible, hey its 2008 and out of everything we've seen so far, the only thing to expect is the unexpected.