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Dude, really good body shapes and details. Doesn't look stiff as much as my drawings. I would recommend trying to draw the head smaller. Although I am pretty bad at proportions too so take this with a grain of salt.

For getting the right proportions I would suggest you draw a vertical line mark its ends and the halfway mark and compare it to the reference you see. you can even draw any basic shape to encapsulate the whole body like a triangle or a rectangle and divide it for better landmarks. Refer to shapes created by negative spaces to correct the pose.

And if and only if you feel like you want to get them perfectly I suggest you to start with a skeleton and then put muscle on it.