I don't know what body this is on, but if I had to guess what's going on with the arms, I'd say they used the Clone body so the costume and armor don't look too bulky, which means the arms are skinnier than regular figure bodies, and Boba has no elbow or bicep pads to hid that. With the arms in certain poses, it looks like the material is being pulled tight, showing the true width of the arms, but in others, the sleeves look fine.
I don't think they mis-taylored the outfit. Boba Fett was a skinny guy in a lot of layers of costume anyway. If you look at screen shots, the inner sleeve looks pretty skinny, while the outer/upper sleeve looks loose and fully. Like gilactiboy said, it's probably just a futzing issue. Stuffing the sleeves may do the trick. I'll look into it when I see this baby in person tomorrow.