But Eddie is a skinny nerd. He was beaten by his brute dad who was bigger than him, as well as brute bullies. Therefore, Eddie developed this hatred for brawn out of his trauma. Hence, why "brain over brawn" is so important for him. It's bad character design to have a character like this to be muscled. Not to mention, it gives him reason to fuel his hatred for Batman, as Batman is yet another "abusive big bully" who beats him, at least to Ed, he is. Batman reminds Ed of his father, and his bullies. Which makes his hatred for Batman stronger, and even more personal. Ed usually cannot fight, or at the very least, he cannot fight well. Having him be skinny also shows demonstrates this. If he's muscled, it's bad character design because it does not convey this as well as it does if he's skinny. Ed in quite a few depictions has also expressed his disgust with muscles. Therefore, he wouldn't want to look like something he supposedly hates. He already has self-confidence issues as it is.