I'm no dietician, but here's a couple of facts/myths/urban legends I've heard about nutrition. No idea how true they are, but they sound plausible to me:

* carbs digest into glucose

* too much glucose in your blood triggers insulin production which turns glucose into fat

* a glucose spike can trigger an insulin spike which leads to a glucose dip an hour or two after lunch

* eating fat triggers the feeling of being full, reducing your desire for more food

I have no idea how true these all are, but my impression is that there's at least some evidence supporting these. And if you add them all up, it sounds to me like you get fatter from eating carbs, because that can lead to a glucose spike -> insulin spike -> glucose dip which makes you want more sugar to get your glucose back up. Whereas food with more fat may make you feel full and stop craving more food.