Good job! Thanks for sharing with us.
I think the detail in the head, both the face and the bubble visor, is well done. It's convincingly realistic and the proportions look right to the original art.
The legs do stand out as lacking detail. They look like sweatpants, whereas Marathon uniforms seem more form-fitting. Wrinkles are unnecessary to my mind. I have to admit, though, my mind's image doesn't always match the actual sprites.
While the marine's shoulder pads are substantial in the sprite, they fit smoothly over the surface. He's wearing boots/shin guards that come all the way up to the knee in the front. The breastplate seems to come all the way down to the waist in the middle. His neck is fully covered up to the chin strap.
I think his build is fine. Looking at the sprites more, I notice the colored stripe running up his sides actually has a slimming effect. The legs half looks normal enough. The torso half is more open to debate on what's armor and what's muscle. He doesn't look very muscular, which is a bit odd given the way he's described.