I have lost almost 400lbs and I feel so different it's shocking. I'll try to explain a few ways:
Energy Level: I was different than most people who are super-morbidly obese in that I had quite a bit of mobility and could still drive, do my own shopping, and those types of things, but I had almost no energy. Even the smallest activity would wind me. Walking across the living room was a tiring proposition. This type of all-consuming inertia doesn't just change how you behave, it literally shapes who you are. Eventually, your interests, desires, ambitions, and preferences mold around your ability or inability to even perform basic functions. Losing weight, I gained more energy every single day. I would literally wake up and think, "I think I'll just go walk outside because I fucking can." I remember there was one point I was down a couple hundred pounds, I was walking to my car to get something and I just had this burst of energy and excitement from feeling my body moving so much more freely that I took off running and ran all the way to my car. It was the first time I'd ran in probably 15 years, and I just started crying when I reached my car. I've never told anyone about that.
Trivial activities: Tying my shoes, clipping my toenails, taking a shower, getting the mail, putting on a belt. These are trivial activities that most people don't give a second thought. They were a nightmare for me. Ever get out of breath while tying your shoes? I have. I eventually just stopped. I tied my shoes loose enough to slip on and off with minimal effort. Now I take joy in doing small things, because it was a casual joy that had been robbed from me for too long.
Clothing: Being overweight limits your selection, sure, but it also changes the way everything fits. Every obese body has the potential for being shaped completely different. Finding shirts and pants that fit (let alone well) was a challenge. It was pretty much a game of putting on an outfit, standing in front of the mirror convincing yourself it looked ok, then spending the rest of the day worrying about which parts of it were riding up and exposing your fat. The first time I could pull a pair of jeans all the way up, and felt good about myself in a dress shirt, it literally made me stop and wonder why I'd waited so long.
Work/Social: When I was super-morbidly obese, people stared at me. There is an innate reaction, and I understand it. At work, and in social situations, there is an underlying disrespect. Not the kind of disrespect you get by being an asshole, but the kind of disrespect that comes from a mix of pity and disgust. Being thinner has demanded more respect in my professional and personal lives, and the process of losing weight before their eyes has increased it as well. I'm not overly-sensitive to the judgment of others, and, for the most part, people have always been genuinely kind, but seeing people you work with daily filled with genuine respect and rooting you along on your journey, it changed me for the better.
Sleep: Obstructive sleep apnea is a dastardly affliction. I would not wish it upon my enemies. At my worst, I propped myself up on pillows, tried to find just the right position, and would eventually grasp and gasp at a modicum of sleep. For at least six years I averaged between 1.5 and 2.5 hours of sleep at night. Even with a C-PAP machine, the sleep was not nearly as good as the sleep I get now. It feels like a real blessing to wake up after eight hours of good sleep and feel rested.
There are too many differences to list here. I could probably speak for days on the topic. I will say this: the truth is a severe mercy, and losing weight, for me, allowed me to see truth in a new way. When I was obese, I could see the truth about the world and how it viewed fat people, but losing weight allowed me to see the truth about myself, and how I viewed the world. I am better for it.