>Some will of course argue that you losing weight will also make you more confident

Having been one of the people who experienced this (well the inverse, scarily skinny to lean and muscular), the confidence comes entirely from people in your life congratulating you, followed by strangers and new people just having a baseline positive glow towards you.

I don't know who came up with that line, it's repeated a lot, but I am almost certain it came from someone who never experienced the transition and soothed their ego by telling themselves it's all just a state of mind.