I think it depends on what you mean by overweight. Some girls are thick, or big-boned (or whatever you wanna call it) but healthy. Some girls are athletic and healthy, others are skinny to the point of unhealthy, and some girls are overweight to the point of being unhealthy.
For the most part, most of my friends, myself included, would rather have a "healthy" gf, than one who is skinny, or overweight. What matters alot is personality, yes. But so does appearance. I'm sorry but it does. Overweight people have more risk of having all kinds of medical problems later in life. (heart conditions, back problems, breathing, etc.) Being underweight also carries many health risks. I think that unconsciously factors into our minds. Also, it can be (obviously it's not a rule but it CAN be) an indicator of lifestyle, like whether a person is lazy or active, neat or messy, etc.
Really, I think it's unfair to blame men on "letting" society shape our minds of what attractive means. IMO the standards were much lower before the women's rights movement. Now, at the height of power that womanhood has ever known, the standards for being an "attractive female" are also the most limiting they've ever been. Secondly, if the standards truly are created by men, then it's because that's what men want isn't it? Not because we let society make us want that. (I'm probably rambling at this point, I hope this all makes sense in some way.)
Besides, TV and magazines really don't reflect the true perception men have on beauty. Rather, they offer a narrow sense of it. I'd prefer an all natural soccer-chick as opposed to some silicone-injected bimbo who's waist is 1/18 the circumference of her chest.
Also I think women promote this trend just as much as men do. I always hear my female friends commenting that being in shape, and dressing in particular ways makes them FEEL confident. Which makes them feel more sexy, which makes them feel better about themselves. And let me tell you that I, and most people I know, find confidence an attractive trait.
So I guess I'd say weight is not necessarily an issue, while being overweight can be, jusy as being underweight can be an issue.