Speaking as a plus size woman myself...no we dont want to look fat!! No, we are not deluded as to our size, but there are certainly some poses that are more flattering than others. Same way some poses are unflattering on a skinny girl!!
Tall bell was right....go up a couple of steps, its flattering to most everyone to be on a slighter higher angle, great for the chins!!! Pose them next to large scale items/architecture - they will seem smaller.
Instead of having someone stand directly in front of them, have them in front and to the side a bit ( ie kids), directly in front will show their size cuz you will likely see both sides of them, but if just off to the side, if will hide half and appear smaller.
Often,if both b& G are large, they will not be able to lean in really close to each othe'rs faces for closeup shots. Have them face each, lean in, then turn toward the camera...its quite flattering and gets them really close! I had a very large groom on the weekend...bride was plus size as well, but he was a REALLY tall, really big guy and it was difficuilt to get them in close. Fortunately, because he was so tall, he could stand behind/to the side of her and lean down over her shoulder for pleasing photos.
Try to avoid sitting poses if at all possible, especially for short/round people. Generally taller heavier people can take a sitting pose if they are more proportioned.
Strapless dresses are the most popular right now and there is a good chance she'll be bare armed...if she has a shawl/wrap, she'll probably want to use it in most shots, so be very aware of how it is sitting. If she doesnt have a shawl, then she is probably very comfortable with the way she looks. You can always use a little liquify tool to the bare arm shot to clean up 'excess'....hell, i do it on pics of myself! ha ha Not to make the arm look skinny, just to smooth out the extra lumps that are usually around the elbow.
Good luck...just be sensitive to belly bulges...please no bride on full profile if there are bulges...and watch the double chins!!
And last, but not least.....you are not a magician, you do not have a magic wand in your hands, and reality is, they are not skinny and they know it, and if they are upset at how they look in pictures, its really not your fault. (well, for the most part, ha ha).