Ignoring the size-ist comments...
Based on my experience as a woman who has been many ranges of the weight scale - here are my thoughts.
You'll likely find several kinds of brides. (Note I don't say heavier, it applies to anyone of any size!) Those who accept (and/or love) how they look, those who don't fully accept it but expect to look the way they will, and those who have a major disconnect between how they "feel" they look and how they actually look. You could have a size 2 bride who will freak out over looking heavier.
I'm sure you understand that there are flattering poses and unflattering poses. As long as you are aware of that and stick to flattering poses and making the bride look her possible best, you meet or exceed any of your responsibility, in my opinion.
Now, that may not be enough to make the third group I mentioned happy. Rather than bashing this group of people (who may not even be fat), be gentle with them, for they are really their own worst enemy and it's a hard battle they fight. Try to focus on the emotions of the day and compliment them often, SINCERELY!!!! Don't make it sound like you are acting any different with them than any other bride. Tell them to relax, that you will worry about the details and posing, to help them be less self conscious, and if they are anxious then work on photos where the couple faces each other, not you for a bit.
And try to remember that beauty doesn't have to mean thin, it comes in all sizes. You can say a woman is beautiful even if you can't call her thin. If YOU (general you, not OP) can't see the beauty in her on her wedding day, you shouldn't be photographing weddings, stick to fashion models.