Screw you, people who tell other people that they can look however they want if they work hard enough. Even if you mean well. It's not encouraging, it's condescending as fuck, and not to mention utter bollocks.
I am small and hourglass shaped, and I am never going to have an athletic build. Sure, I could be a little slimmer, but I'm fucked if I'm going to choose loosing a bit of fat from my stomach over eating nice food and enjoying myself. I am almost certain that, even dangerously underweight, I would not look skinny. I literally do not have the height for it. I am the chibi to other, taller women's slender anime characters, and I'm (mostly) fine with that. There's damn all I can do to change it.
Oh, and especially screw you if you're telling me this being substantially taller than me (which is most people), or with a magical metabolism. Go fuck your self control, I'm already eating substantially less than you. You may think that you're helping someone out by telling them they can achieve anything, but what you're really doing is making that person feel like they're being judged for looking like a regular, healthy person, and reinforcing damaging beauty norms and myths about weight and health. Or you might just be an arse.
ETA: Can I just say how delighted I am that this post has evolved into a conversation about bacon-chocolate-cannoli. I love you GT.