There's some truth to this, and I'm sorry it's been your experience, but I'd like to gently expand on this a bit as I don't think muscles are the only thing that matter - I know plenty of skinny and fat people with friends.

Relationships are inherently transactional. You won't want to spend time with someone if you don't get something out of it, barring certain unconditional loves like your immediate family. When making new friends, proxies like attractiveness and social standing are how people judge if someone is likely to add value to their lives or not.

So yes, unfortunately, if you talk to someone and you're just some small quiet guy with no interesting characteristics, you'll probably be written off before you get a chance to develop a friendship with them. Whereas if they see you have muscles, or know you're successful in your career, or know you have other friends, they'll be more likely to assume you might be worth getting to know.

Things like working out, dressing well, learning to speak well, etc. all help. However, there is an alternative shortcut to building close friendships - forced interaction. When you're stuck sitting next to someone in class for a year, you don't have the privilege of swapping that person out for someone who seems cooler, you just have to get to know them. When you're stationed in the military with a squad you don't get to swap that squad out for people you think you might like more, you just bond with them. But there are few opportunities like that in normal life, you have to seek them out. Go on a 2 week long canoeing expedition, join a start-up incubator with a small team, play an MMO at a competitive level where you have scheduled runs and are in voice chat. Do stuff that forces you to interact with people for a long time and puts you in environments where you can't just leave and seek out people more like you.

People will want to be around you if you’re kind and generally pleasant, which is a much stronger reason to hang out with someone and leads to much longer lasting and healthier bonds than a physical trait. It also costs nothing and once you can get into that mindset, takes no effort.