First off, I want to say great job on your suit! While I know there are things you wish to improve, there is so much of quality here that I want to congratulate you on your amazing work.
As far as issue are concerned, a lot of what I notice are just general scaling issues. This is something that has plagued so many of us with our builds so hopefully I can provide some insight as a person who has experienced very similar issues with scaling in the past.
When looking at this build, as others have stated, the first thing noticed is the chest/torso piece. It seems that 2 things are occurring here to me. First the scaling is just a bit off. I do believe it is ever so slightly over scaled and this is causing it to appear quite large in comparison to other parts of the armor. A slight adjustment down could correct this in a big way. It also appears to be rolling forward ever so slightly. This is giving the piece an odd appearance as it seems your trapezius muscles are inflated. Some adjustments to your rigging could correct the rolling issue and provide a bit more support (and comfort tbh).
The next issue I see is your bicep armor. Again, the biceps seem to be a bit overscaled compared to other parts of the armor. A small adjustment down in scale and then raising them ever so slightly would go a long way here.
The last thing I would want to touch on scale wise are your legs. You did an awesome job scaling these to your exact proportions. The only problem with that is, our MKVb buddies are CHONKY! Becuase of the other scaling items we touched on above, your legs appear very slim in relation to the rest of the suit lending to the overall appearance you described in your initial post. If we look at reference material around MKVb armor, we can notice that the thighs and shins are often very wide. This leads to the appearance of a spartan who definitely didn't skip (and probably did extra) leg days.
So now that I've touched on all the issues I see (and feel bad having written them out >.>) let's talk about what we can do in the interim without you having to reprint a ton of pieces.
The first thing I would suggest is to adjust your rigging around the biceps and chest plate. If you can manage to ensure the chest isn't rolling forward and the biceps are sitting a little higher, I think this will help lend to a more balanced appearance. I would also every so slightly adjust your belt and thighs when doing this as well. Rolling the chest back a bit might require some adjustment to the belt and thigh pieces and my suggestion would be to ride them just a tad higher.
To fill gapping between the chest and biceps, I would encourage you to look into crafting the shoulder undersuit pieces if you can. These will make your shoulders appear larger and will work similar to what Cadet said above in regards to a muscle suit. This will avoid having that void of space that you are noticing currently.
Another thing you could do is craft out the ab wrap section. This would add some bulk to your mid section and help make the chest appear a bit smaller.
Overall though, be proud of what you have. This is a great looking suit and I look forward to seeing what you do with it in the future!
