This is very common with the MOBs but sometimes, when this is said... it is true.
If someone is going to try to do portraits with a sub-70mm lens, there *is* a lot of distortion (most easily seen in the face), that comes up and looks like someone gained 30 pounds between this picture and the last. Using a long lens (I just crank it up to something between 100-200mm), more distance and compresses the picture. This makes people look a lot slimmer and gets rid of what I call "chipmunk cheeks".
Now, there will be times that no matter what you do, a person with excess weight will look like a person with excess weight and they may ask you to play with the pics. This can be a time consuming process and I believe in charging the client for your time spent adjusting these pictures for them. Same goes for scar removal, acne, etc...
As a general rule, I would do some skin softening and cleaning on closeups of the bride, but beyond that, what everyone looks like, is how they will appear in my pictures simply becuase the day is NOT about them, but about the bride and that is who I am working to make look good, no one else... unless you wish to compensate me for the extra time to make Aunt Jenny and her 50 pound overweight frame to look like Marilyn Monroe... lol. These are the shots that one could spend a lot more than 15-20 minutes per picture on.