By no means I am overweight or fat, I've got defined abs and I just got my twice a year stats back (this is mandatory since I'm a sperm donor). It says I am 8% bodyfat and my knee cartilages are fully recovered.
Maybe its because I used to compare my body to models and pictures of perfection?
But out of all seriousness, I do understand where you're feeling 1st hand. When you are more active overall, you feel what natural posture and energy flow is supposed to be.
When you sit, you notice that your posture doesn't feel "natural"...because it isn't. You feel the pressure on your obliques, abs, and lower back, as well as your back and neck, and you feel lethargic. You feel the pressure on your back and hips, and it doesn't feel "natural", as if your energy flow is disrupted (because it is!).
I remember back in the days years ago back in my dorm years of college, when I would be content sitting and laying down (playing video games, watching TV, on the computer, etc.) for most of the day if I had nowhere to go.
Now...im definitely more antsy. I can't sit down in one spot for too long. Sometimes I eat standing up. Even when I am on the computer, I rotate between sitting and standing.
I eat mostly foods that look like the ones in my pic, if you eat heavy cheesy, greasy, foods no matter you're weight they will make you feel heavy, also your bowels may be backed up if you regularly eat these foods, maybe try a colonic!
I am willing to say that a great many of those "models" have been altered. Even if only slightly. I completely agree that I feel fat sitting down though. I am back down to a size one POST pregnancy and I gave birth 3 weeks ago! lol. It is just a psychological thing though. I am in no way over weight and I kno that. If you sit up nice and tall it doesn't reduce the small amount of "fat" that is there.