This is phenomenal, my only critique is the TINY head. Tiny head is TINY. It's even more redenkulous because the rest of the proportions aren't too far out (keeping in mind this is 40K and Space Marines which laugh at such nonsense as body proportions). Could you revisit this sometime if you have the time and make that head right about two times bigger? When I have the time I can try my hand in manipulating that pic a bit but I'm afraid of defiling your work. Keep up the good work!
When people draw Space Marines, or those who make jokes about the proportions, don't fully grasp the whole genetically-enhanced thing. Space Marines and Primarchs are massive beings: large, long arms and legs, broad shoulders and a large head. There's a post going around on tumblr in the 40k side of tumblr that brought up this question.