one thing that's throwing a lot of the proportions off is the neck, which appears to be extra long because of how low the pauldrons are from the head. His trapezius could use more 'beefyness' going into the shoulders and if you raise up the shoulder bits a tad it'll look more like there is a form inside the armor. This same type of problem can be seen in the torso, which might fit a lanky guy in it maybe, but generally armor wouldn't be that snug of a fit. How would the guy get out?
I definitely agree about the left arm and maybe the right arm has a bit of wonkiness to it, but I can't really tell because - as arran stated, the legs look especially small - though the camera angle slightly contributes to that. Check the proportions to an orthographic reference or something. Don't forget that there's gotta be a person inside of it
Overall quite well done, I would have liked to see you finish