Tonight the Beta 19 should fix the issue Sajmmon reported some time ago. It was due to a wrong memory management that could depend on Java behavior. This same correction should fix also some image texture clones that were generated by OBJ export from time to time.

In v4.4 you are limited eta=1.33 that means I can't render diamond(eta 2.415),flush blood(eta0.8)?

Finally, I forced eta to 1.55 for a Ni value smaller or equal to 1.
As a value smaller than 1 is not realistic, and that a Ni value of 1 is actually wrongly generated by Blender export, I decide it was more logical to use the index of refraction of the glass 1.55 for wrong values in Sweet Home 3D, a program used to represent furniture, windows,… as I may recall you. [;)]

At a given quality level, you can still improve the image quality by choosing a bigger size for your image, then reducing the size of the computed image in an editor like The Gimp.

In these lines you are talking about to change DPI/PPI of the image?

No, just resizing it to a smaller size. You can set an other DPI if you want too, but that won't reduce the file size of the image. Anyway there's no DPI information in images generated by Sweet Home 3D.