As portrait photographers we're always working with an ideal of beauty on some level or another. In life, these standards of beauty can be oppressive, damaging, but even the most independent and rebellious person tends to have at least one part of their body that bothers them because it doesn't fit that ideal. Weight is one of the most common, and while every intelligent person knows it doesn't define beauty, most people would rather not see their weight "issue" immortalized on film. And this is true whether their weight issue is real or not. Couples can see beyond to the real beauty of a person, but when two people are pictured together, their difference in size can become even more obvious.
There are a number of techniques to "balance" a couple on film. Generally speaking, each person should take up about the same amount of space in an image. Culturally, we can see the man as a little bigger, but surprisingly they don't look any less masculine when equalized or balanced. And if the man is a lot bigger than the woman, it can be equally distressing for them.
In standard V pose, couples of roughly equal size photograph "well." It looks like this:
(You could have her face the camera directly and him angled towards her-- and this can work well for a certain feel-- but now he's in the feminine pose and she is in a more masculine pose.)
If she is larger than him, he can face the camera directly and she can slip her arm through his, placing her shoulder behind his.
Recently, I photographed a couple who's height difference was 1.5 feet. We played with her on a curb and him below, and sitting in various positions. Her in his lap but their eyes level. One thing you never want to do his have him slouch-- that'll ruin a picture.