Obviously, yeah, someone can be both fat and attractive. I mean, look at someone like Queen Latifa. Is anyone going to say she's *not* a beautiful woman? She's a spokeswoman for a cosmetic company, for goodness' sake. They pay her big bucks for her pretty face to promote their products and she's definitely gotta be classified in the obese range.
But, a couple things. A lot has to do with a person's proportions and how they carry their weight. For instance, I myself unfortunately bloat up in the face when I am even slightly overweight and because my features are small and delicate to begin with, I lose whatever facial prettiness I have. Other people don't get fat faces even when their bodies are really heavy OR they have such strong features (huge eyes, heavy bone structure) that gaining weight in the face doesn't affect their looks (or actually makes them better looking).
Secondly (and unfairly) I do think that fat people have to put forth a little more effort with their appearance to be considered attractive. A fat woman who has a really nice haircut/color, wears some tasteful makeup, and dresses in stylish clothes is probably going to be more positively rated on the attractiveness scale than one in frumpy sweats, no makeup, and stringy hair, whereas one who fits more with our societal conception of attractiveness (let's say thin, tall, blond, and young) will probably be considered pretty by a lot of people even if she's in the supermarket in pjs, with greasy hair in a ponytail and unshaven legs.
Attractiveness as in
sexiness is a whole nother matter. Some people, no matter what their appearance (thin, fat, tall, short, ugly, pretty), just exude something charismatic that makes other people wanna be with them.