So there's no way to "fatten" him up? That's too bad. I think shortening him a bit (even if just bent legs) and giving him a little girth would add much to the variance of shapes among the bounty hunters.
You could always just shove something up under the figure. Add a little padding and just tape it into place. It aint elegant, but should do the trick. Height, for myself, doesn't bother. She is clearly shorter than the rest, and I am ok with that.
Ironically when I got my HT Jyn, I was bothered by how tiny the figure was, though I am sure it was in scale.