I don't know what you are doing so I tested it myself. Here's a 640x480 72dpi image rendered in Poser 11 using Firefly.

Here's the same image rendered at 300dpi in Poser 11. All render settings the same, except the DPI setting.

The two images are nearly pixel-for-pixel identical.

And to be clear, here is the setting I changed:

When you rendered your two images, did you keep the width and height the same, in pixels? If you are rendering them at different pixel dimensions, there will obviously be a quality difference. But at the same pixel dimensions, there is not.

If you have changed the width and height units to something other than pixels, then when you change the "Resolution" settings to a different number of pixels per inch, you will get output with different pixel dimensions.