i haven't touched Poser 6 in years, but have you tried to export as .jpg at 100% quality ?

Or .TIF or BMP ?

And then convert to PNG later ?

Also, if there is no proper background in your render, a lot of the PNG will be transparent, so the file size is actually quite small.

Can be even smaller than a jpg, because there is no transparency in a jpg, so even an empty background would export as a solid color.