Yeah, I do agree that the chestplate was pretty big. I was also looking at where you have the side of the chestplate at around your rib cage and it looked like it was sitting a tad too low.
Also, I think it looks off because of the file you are using. There are better, more accurate chestplate files out there. I believe the one you are making was created BEFORE the game's release and now that the game has been out, people have made more accurate chestplate's. L3X has an AWESOME chestplate and is nicely detailed. It's also a bit more accurate than the one you are making. I would suggest checking out some of the more accurate chestplates.