The used texture depends on the quality of your images.
After texturing you can check the texture quality by selecting the model.
If the texture quality is at or near 100% and it does not fully use the available texture space then there is no point in havig such a large texture. Make it smaller. RC is giving you all it can.
If your texture quality is way below 100% then you may have to redo the unwrap and make sure to check the optimal texel size.
Set texturing style to fixed texel size and use that value.
That will make it use pretty much the whole available texture area.