Well- I can see two different possibilities here, and maybe they can occur at the same time.
One- yes, absolutely it's annoying as crap when other people decide to be your diet/exercise police. That's irritating. You can tell them to knock it off, if you can figure out a way to do it in a manner that preserves the friendship (assuming you want to preserve the friendship). There's another
thread here where we kind of discuss that and how really people should keep their mouth shut unless
asked.
Two- if these people are close to you, they could be doing this out of love. Are you overweight, or are you obese? Do you really exercise? Do you eat crap, or do you eat healthy? If they're close to you, they'd know as well as you do, and they might be more realistic about it. In today's culture, overeating is so simple to do. I was seriously overeating. I ate a lot of healthy food, but I ate a LOT of healthy food, if you know what I mean. But I'd have one plate and think, that's just one plate. Well, yeah, but it's a 12" plate. And I thought I was active, until I got a fitbit, and found out I was doing somewhere between 4k-5k steps a day. Maybe this doesn't match you at all. Maybe you go to the gym every day, and you do eat healthy, and you're only a few pounds over where you should be and your friends/family are judgmental punks. Or, maybe they love you and you're not being realistic. I just suggest you think about that and be honest with yourself.
Finally- absolutely someone of a healthy weight and exercise level can have a different standard for you if you're not. My husband is lean and healthy. He eats pasta, pizza, etc. I don't eat those things, because if I do it goes to my belly, butt and thighs. He plays volleyball once or twice a week, I work out nearly daily. Now to be clear, he's not pushing me to not eat something or to work out more (which does make it different from your friends) but he's also not trying to match my level of exercise and eating habits because he doesn't need to. Maybe your friends work out in the morning before work or after work and so they don't need to take the stairs, but they love you enough to do it anyway. You know? I guess as annoying as it is, try to think of what their motivation is. You may not be as healthy as you think you are. Or you may need better friends. Only you will know but try and be super honest with yourself about the answer.