The armor up till now is looking great, but agree, the chest piece is still not quite right. The one thing that makes the marine armor look impressive is the physically imposing size. I agree, the chest plate needs to curve out more.
However, looking at the miniature directly above the post, you also need to create a taper on the sides, making the piece broader at the shoulder level, then tapering towards the waist, which would probably accentuate the bulk of the chest plate.