My monitor accepts 1080p signals via the HDMI port but its native resolution is only 1680x1050. When I watch Blu-ray movies on my PLAYSTATION 3, who is resizing the image? The monitor or the PLAYSTATION 3?
I'm pretty sure it's your tv. The quality of the resized image depends on how well the tv hardware/software does the down conversion.
I just realized that you said monitor instead of tv. When you said HDMI I assumed it was a television. What monitor do you have?