Hi
Anecdotally, I’ve noticed skinny people don’t get tired that easily.
Is this really true?
If so, why? Are they skinny because they are energetic, or energetic because they are slim?
I’ve gained a lot of fat recently, and I need as much motivation as possible
My sister was a vegetarian for all her life and she was not just thin, but gaunt, and sickly in the bargain. She checked out at age 65 with colon cancer which is supposed to come from too much fatty meat, ain’t it?
To some degree, certainly. It takes energy to lug all that weight around.
Everyone I know who has lost weight (except due to chemotherapy!) remarks how much more energy they feel they have. They’ve just taken off a ten-pound weight belt! Of course they feel more energy!
Given that the body stores excess energy in the form of fat, wouldn’t a fat
person have more energy than a thin one?
Yeah, I reckon it pretty much depends on if you want to use them as fuel to keep the house warm, or if you want to run an endurance race against them. 
This is both why I love, and why I hate the SDMB.
I can only speak for myself. Yes, for me this is definitely true. Until my early 30s I was always thin. Then I had two kids - one at 32 and the other at 36. I only gained 25lbs with the first and about 30lbs with the next (which I lost, mostly), but somewhere after my 2nd kid, I established bad habits - eating larger portions, too many sweets, breads and late night 4th meals. That, along with not having regular workouts, ballooned me to what I call – “my maximum density.” One day I weighed myself and realized that I weighed as much as in my 9th month of pregnancy - 163lbs. I am 5’2’’ and have a medium to small frame. My cholesterol was super high, too. It was a wake-up call for sure. I got serious and within a little more than a year lost 43lbs. That was over three years ago, I’ve kept most of it off. I’m around 126 now, but my energy level is still absolutely higher (even at 50 years old). What carrying all that extra fat did for me was make me tired all the time. My knees, feet and back hurt often. Pain sucks energy out of you. Also, I discovered that workouts actually gave me energy. Going back to my old weight brought a lot of positive changes in my life that went beyond physical appearance and health.
With that said, I have met overweight people who seem to have tremendous energy. I’m thinking of one friend in particular - she’s in her late 30’s, she’s a farmer and is always on the go. I’d say she’s carrying about 40 extra lbs on her. Then there’s a woman at my work. She’s naturally skinny, has very little muscle mass, but she tires easily on simple hikes with her family. She never works out. Ever. She’s my height and about 110lbs, if that. I’m 12 years older than her, but I can guarantee you that I have more energy than her. So who knows…the answer to the OP’s question may be a very individual thing.
They say that being just ten pounds overweight is like lugging a big bag of potatoes around with you all day. It isn’t, really. It’s usually more like setting it down on the couch and leaving it there for a while.
There is a grammatical irregularity in that sentence. 
(Total agreement, by the way!)
Is being not skinny the same as being fat? Or is being not skinny being heavier but still in shape?
Is skinny being in physical shape or just a body frame with a lack of muscle?
I don’t think there is enough information to decide one way or another.
Is a skinny person in shape or just someone who is ‘bones’?
Is a heavier person out of shape or do they just have more muscle mass?
What activity do you have in mind?
A fit 110 pound MMO fighter would struggle against a 150 pound MMO fighter. A 110 pound runner might do better in a multi-marathon versus a person who weighs 150 pounds.
I’m inclined to think that thin people are thin because they have more energy. There’s a woman at work and she is never still. Thin as a reed and a dynamo. She wears me out just watching her.
I lost a decent amount of weight in the past several months and I find I have have a great deal more energy. I am not thin but I would venture a guess the answer to your question is Yes.
The more I do, the more I want to do.
I once weighed in at about 160 lbs. I honestly don’t know how I carried it around. These days, if I allow myself to get up to 130, I feel grossly obese.
Shitty lifestyle that makes you fatter (lack of activity, lack of exercise) can decrease your stamina. So can sleep apnea, which is tied to obesity. Plus people get fatter with age and kids generally, and that can sap your energy.
I’ve read a theory that one of the reasons thin people are thin is because their bodies aren’t too good at storing extra calories as fat. If they eat too much the calories go to the muscles instead, which causes the person to become more active to burn it off. Their bodies can’t just store it as fat, it gets taken to the muscles instead.
I’m sure they all play a role.
I’m about 5’4" and 107 lbs or so.
No energy. I may just be an outlier, though.
Some thin people are thin because they are sick. In that case, they wouldn’t have more energy.
Some thin people are thin because they have naturally high metabolisms. I don’t think this would necessarily correlate either positively or negatively with perceived activity level. But I dunno.
Some thin people are thin because they don’t eat a lot. Like anorexics or people in famine. Or health-conscientious people. I wouldn’t think this group would necessarily have more energy than anyone else. But for psychological reasons, they may be able to push themselves more.
And then some thin people are thin because they exercise a lot, which means they are more certainly more “energetic” than most people. But what comes first–the energy or the motivation–could be anyone’s guess. I think in my case, it’s a little of the first and a lot of the second. I’ve always been kind of a fidgety individual. But I am certainly not “high energy”. I don’t bounce off the walls (without caffeine) and I’m not always in an “up” mood. Since I’m chronically anemic, I probably get tired more easily than most. But I am motivated to override that initial sense of fatigue because for me, moving my body is a great way to release tension. Tension makes me feel much worse than fatigue does.
Lacking energy or motivation are not good reasons to feel bad about yourself.
I’m inclined to believe they are thin because they have energy and therefore move more.
High metabolism may lead to thinness.
I developed sciatica when I was pregnant, and couldn’t walk much, but I figured I’d have the baby, and it’d all word out. A year later, I still had terrible back pain, and couldn’t get much exercise, and my weight was creeping up. So I started PT. I was nursing, and couldn’t take anything stronger than OTC pain meds. I got a little better, but sometimes I’d still get bouts of it. Finally, after the boychik was weaned, I had steroids and tramadol, along with more intense PT (and my breasts were supposed to get smaller but they never did).
Anyway, it took a long time to resolve the back pain, but one it did, I lost some weight, so there’s definitely a feedback loop, but you can’t make assumptions about what overweight people may feel motivated to do, vs. what they physically can do. When I was in a lot of pain, there were plenty of things I wanted to do, but my back hurt too much.
Interestingly enough, one of my best friends is gearing up for bariatric surgery and one of her handouts from the hospital suggests (paraphrasing) that she learn to “fidget” more. We talked about that a couple of days ago and she pointed out that I was a very fidgety person and annoying to go to movies with because I don’t sit still. I am fairly skinny and always have been but I think my energy and activity level is between average and a little higher than average, but not frenetic.
I don’t know which is the cart and which is the horse, though. I can also think of a few people I know who are overweight, but have tons of energy and are always on the go.