They're appealing to their target audience: the lowest common denominator. In this case, it's fat people who are feeling bad about eating fast food. They see the lovey model eating the burger, and feel better about themselves. Then they go and buy 10 Big Macs from McDonalds since they forgot which place was doing the advertising.
It's disgusting on multiple levels, but that's what the majority of advertising is. It doesn't help that, as far as media is concerned, the big issue for women is body image, and some of their biggest advertisers and influencers are fashion companies. I wouldn't be surprised if in those images they were trying to sell "lovey fashion" towards females as well.
EDIT: To make it clear, a lot of companies are amoral, and they don't consider people as people at all. They're viewing them as objects that, if strategically placed, will give them the best profit. It sucks that it's the usually the females who draw the short end of the stick, but everyone is being played by these corporations.
