I agree with trevoriuS, the arm needs some fixing. It looks pretty awkward and even though a chaos marine might be capable of easily lifting a cadian (looks cadian), could probably pull it off with a sword but would not be using such lack of form. He's not a berserker of Khorne. Tzeentch's men usually prefer long range weapons, spells and the colors blue and yellow. Tzeentch also has a huge crush on egypt and just about everything egyptian. Or at least that's how I interpreted everything.
@TrevoriuS:
Space Marines are pretty big ( genetically altered) guys to begin with. Now imagine a big (genetically altered) guy encased in pretty awesome armor that not only increases your strength but it's pretty strong. Now imagine this hulking (genetically altered) brute in armor tainted with a demonic force that instills various powers and possibly mutations on him. He's pretty big now. So I think the marine and the imperial guardsman are pretty good as far as size goes.
Ok so enough about not pixel art.
I think you need to fix those horns, they seem a bit off, his right one (our left) becomes straight, it also looks very flat. I think his torso needs to be fixed up, I think they're more like spheres ontop of cylinders instead of body shaped. His backpack thing also looks weird, looks more like a slab of metal on his back instead of the backpacks most of them have. You should either replace the cadian's head with a bolt gun or a power fist. The sword arm has a goofy wrist, looks like it can bend/articulate. You should consider modeling it after a gauntlet.
P.S. You can probably view each individual piece that makes up your hulking brute on the games workshop website or fan sites. Good luck, I hope I've typed up some decent information for you. Use google and wikipedia for more info.