Pros
- jiggling feels awesome
- built in padding makes life more comfortable
- people stop expecting you to do active things, be dieting, etc so less pressure to meet those 'good' touchstones.
- people who are nice to you are more apt to be nice, not just sucking up
- shared experiences help bond with other fatties
- plus size stores generally have really friendly staff
- fat faces don't seem to age as quickly!
- you have more physical presence, meaning less jostling, less chance of people unconsciously trying to physically intimidate you, etc.
Cons
- chaffing, especially in hot weather (thighs of course, but anywhere flesh rubs flesh)
- little things get harder (like getting out of breath from tying shoes when a belly is in the way, harder to wax or shave legs, etc)
- less choice of clothes once in plus sizes
- harder to deal with beat, even when not moving
- fitting into seats with arms becomes an issue
- any food you drop lands on your bust or belly depending on your build
- having to find 'wide width' shoes for fat feet
- slower walking
- chance of insults from random strangers
- chance of doctors blaming everything from nose bleeds to ingrown toenails on your weight.
- life gets more expensive (clothes, food, premium seats for travel,...)
9 years
Cons are achy feet more often than usual, though. And buying new pants more often 
