If a guy happens to find slender women attractive, rather than heavier ones, then it isn't a matter of whether it's "supposed to be that way in a relationship". It is what it is, and people (men or women) need to be attracted to the person before they get into a romantic relationship.
It' a big mistake for a guy to think that's the ONLY important factor in considering getting into a relationship with someone; because there are a lot of people of all weights with rotten personalities. Still, people are attracted to one set or traits or another; so I think that's a reasonable "prerequisite". Might the guy miss out on dating a really, really, nice and/or smart person because she's heavy and he isn't attracted to her? Sure. Still, if he's not attracted to her that wouldn't make much of a romantic relationship (and if they dated they'd both probably end up unhappy). Some men, more than others, care about one kind of build or another. They can't help what they prefer. (Neither can some women.)