You can control border/margin size. I don't have RizomUV in front of me now, but it's in the toolbar somewhere. (a bit to the left of the middle, sorry, that's the only reference I got for now)
The other thing that it's not using the entire tile, I think it comes down to the texel density. RizomUV tries to scale all islands as even as possible. I've not investigated this enough, I don't know if it can be switched off entirely.