Yup, noticed the exact same thing!
Here's a comparison of the Martial Artist's Vest and the Martial Artist's Sleeveless Vest:


Vs Sleeveless:


As we can see, the muscles around the shoulder and biceps become much bigger when wearing anything with sleeves.
My guess is that the femgar model is based on one of the male models. The rigging is honestly great from the waist down, but anything that goes over the arms or the shoulders looks unnaturally big.
This also leads to the head looking too small. I think it's actually the right size, it's just that the immediate shoulder area is too big to fit with the rest of the body.

I wonder how hard it is to change the rigging once in place. I would very much like to see the arms and shoulders reduced a bit if possible.